They really are just looking to smell your breath (alcohol?) and, perhaps, figure out if you are acting strangely (“I’m just coming from downtown”. “But that’s the other direction”)
They don’t really care about your answer.
During the George Floyd protests, I recall the black Republican senator mentioning how many times driving in DC he’d been pulled over. Presumably a well-dressed elderly person, but still…
When I saw that, I was thinking that I’ve driven around the USA severla times a year for decades, even on a motorcycle, with longish hair, and the only time I was ever pulled over was due to speeding by a radar cop (who was polite but issued a ticket).
I vaguely recalled, though, that the police had to have some reason for doing it randomly - if it was suggested that racial profiling was the issue, then they had to at least defend against that. Typically, they will set a up full blockade on a road and stop every vehicle to look for drunk drivers in holiday season, and that’s entirely legal since nobody is being singled out. Hence, no ground for a discrimination claim. (I did get pulled over once at a blockade in cottage country, at night, by Conservation officers looking for illegal hunting or overfishing. They let me go right away when they saw the bags of groceries in the back seat - “oh, you came directly from the city”.)
In the U.S. there is caselaw that limits random checkpoints. They can be done but they have to be for a specific reason like DWI. The procedures have to be random (like every third car). It also has to be published ahead of time when and where it will occur. It used to be printed in the newspaper but now it’s usually done on social media.
Immigration checkpoints have different rules.
Can they pull you over for no reason, other than to verify you have license, insurance, etc.?
At least here in The States, an LEO can’t say to himself, “Hey, look… someone is driving a car. A driver license is required to drive a car. I’m going to pull them over to check if they have a license.” The LEO must have RAS or PC the driver (and/or someone lese in the vehicle) has committed a crime in order to pull them over and detain them.
You can be pulled over for any non-protected reason, including no reason. They can demand insurance, registration, license, and a roadside intoxication test. They can’t search you or the car or demand ID for passengers without probable cause.
FWIW, In the Criminal Intent episode “Albatross” , IIRC*, Goren “pulled over” 
a limo driver that was parked “too close to a hydrant”, so he could grill him.
*I hope that was the one. I can see the scene in my head
I never watched much CI. I’m aware Dick Wolf, the creator of L&O was very much a police booster, he intended to show off the cops as good guys, never just harassing people they don’t already have good reasons for suspecting them of wrongdoing. Definitely never any scenes showing the heroes stopping somebody on the street for just WWB (walking while black).
The shadiest thing I remember seeing the cops on L&O classic doing related to a traffic stop is they had an arrest warrant in hand, but no probable cause to do a fine-tooth examination of the suspect’s car, which they wanted to do to gather more evidence. They apparently were not able to get that warrant, so, they waited several hours for the suspect to drive the car out onto the street, and then arrested them. A tow truck waiting, and of course, now they had an excuse to do a proper “inventory” of the car’s contents pursuant to arrest. The suspect’s lawyer challenged the evidence thus gained, knowing exactly what the cops had pulled.
Politely, have you even watched the show.
There were L&O mothership episodes where cops:
killed a guy for DWB (the actual name of the episode), by dragging him behind their car until dead
Set up a gay cop to be ambushed and killed, and slow rolled the backup
Set up a white kid to be killed because he mouthed off to them
murdered her cop ex-husband because he was abusive, and the blue wall protected him
was outright corrupt and lied to get Lennie Brisco to look dirty
Set up an informant to be killed because of what he could say
let the wrong guy go down for murder because the real killer was connected, and the brass said “it wasn’t him”
shot her own partner, because he was going to murder her
and that’s just from memory
That’s not necessarily required.
As stated upthread, police can also pull somebody over if they have probable cause that they committed a civil traffic infraction; no crime suspected.
Additionally, most jurisdictions recognize what is called the “community caretaking” function of the police, which allows them to stop a driver if they have concerns that they might be sick, tired, or injured.
(The thinking being that a cop doesn’t have to wait for an actual traffic infraction if they have concerns about the welfare of a driver. Practically, though, it just becomes a nebulous grey area that police use to justify a stop)
I’ve had police in Canada pull me over for being the only car on the raod in the middle of the night. Another time, claiming I was speeding when I wasn’t. (“No officer, you were parked there and saw my brake lights come on before the 50kph sign”). Many years ago, for not having a front license plate when that was normal in some provinces. One fellow from a small town mentioned that later at night it was normal for the cops to randomly pull over people to check for DUI. They don’t really need a reason, but rarely do they just randomly do so outside of approximately bar closing time.
I used to admire how the cops on Chicago PD managed to rough up or even kill just the really guilty ones, not the innocent ones. (Although IIRC there was one epsiode where they railroaded a uy who turned out to be innocent and died in prison). i see in recent seasons they’ve moderated this - I assume too many complaints.
You know, the same for Anaesthetists and medics. The first point of continued conversation is: “are you breathing?”
“Chicago PD” is actually a spinoff of a different show, IIRC “Chicago Fire”. The lead detective for CPD was an arrogant prick who tried to cover up a DUI of his son who injured someone. He was set opposed to the hospital/ EMT staff who responded to the call, who he tried to bully and threaten. Needless to say, I was uninterested when they made him the lead character of his own show.
I tried a couple episodes, and was turned off. Too unrealistic. Bad acting.
Most cop shows have annoying decided that instead of a good mystery/adventure show, we need to drag in personal life and problems of the police themselves; which often brings up for me the question about how the volatile short-triggered cop ever manages to keep his bage more than a few months if all he does is mess up cases by trampling the perp’s constitutional rights.
Plus, none of them think to cover the back door, and half the time wander through the crime scene without gloves. And when they enter a scene with the SWAT team, they are typically in front but without the shield and body armour - or the high-powered weapons. But my wife gets annoyed when I point this out during the show.
All true. I give that last a bit of a pass , like the Star not wearing his helmet in a medieval period film. They are the stars, after all. I just watched a Blue bloods and they did let the ESU go in first, but then there was a lot opf shooting and perps running everywhere, so the camera follows Danny nabbing one perp, rather than a extra nabbing some other perp. That’s okay.
But the back door thing, and calling out a perps name when they are 20 feet away, thus a foot chase, is just bad.
The worst example of “back door” not being covered was with the Front Door- on the FBI- the two stars go in with a dozen SWAT- and the perps runs out the door they came in- becuase no one covered it. Yeah- of course they cover that door, they dont want anyone else coming in, like more bad guys or innocents that could get hurt. Really bad writing.
Danny from Blue Bloods is an interesting example- yeah, he whups on perps in custody more than is acceptable- but then they say he will never get promoted as altho he has the best conviction rate. he also has the most civil lawsuits.
Psst, Cafe Society is over thataway.
Oops. Sorry.
The question isn’t about anyone pulling anything on cops. It’s about something the OP has observed cops doing without any prompting on his part.
You are replying to a situation where someone is stopped with no justification while just walking down the street. If you are pulled over while driving a vehicle, as mentioned in the same post you are replying to (but didn’t quote that part) there has to be justification that you have committed a traffic violation (or probable cause you have committed a crime). Therefore that’s why you have to show your license, in CA and virtually everywhere else.
I’m not going to disagree with you regarding whether the answer would be presented as evidence, but it seems odd that a police officer pulling me over needs a conversation starter. It’s not a cocktail party. It would be just as effective to tell them flat out why you are pulling them over. It seems like that would also establish authority. Asking the driver comes off like a head game, since it’s obvious the officer knows the answer.