The only time I remember going to a drive-in movie was in Escondido, CA in 1984; the movies were Cloak & Dagger and Red Dawn.
Well, before television (and Maine, for example, had no TV until 1953), it was someplace a young family could go with all the kids, and not worry about being kicked out.
And, before multiplexes, it might be the only place near you with the movie you wanted to see.
And, in the early days, they might have playground stuff, or even animal rides, open before the movie started.
And the whole idea was intrinsically fun.
And, of course, it offered a greater amount of privacy for teenagers than the legendary “last row in the balcony”.
Yes, and in that sense the question “Why would people go to a drive-in to watch a movie when they could go to a movie theater?” is a little bit like “Why would people have a picnic when they could eat in a restaurant?”
Yes, watching a movie at a drive-in as opposed to a movie theater had/has some of the same advantages as watching one at home. Except that in the 50s, 60s, and 70s, the only way for most people to watch a movie at home was to wait until they showed one on TV and watch it on their small, possibly black-and-white screen.
And when you woke up the next morning…? 
Son! Is that you? 
There’s still some around, including six screens in just Riverside! Here in San Luis Obispo we have one just over the hill from my house; a half mile as the crow flies.
One thing that hasn’t been mentioned is the use Drive-Ins are/were put to on weekend mornings as very low end swap meet/flea markets. When I was a teen in the 1970s I remember walking away with a Stratocaster knockoff, 3 bowling balls, and a stack of Playboys for less than $50.
Yeah, they’ve made a come-back for some reason, and I would imagine the ones that weren’t plowed under could be retrofitted for better equipment and for not a whole lot of money.
Yeah, with digital projection, I imagine the cost of equipment is a lot less and can probably fit in a closet with little attention from the minimum wage operator who can scoop popcorn and clean restrooms during the show.
Yeah, and keep in mind, movie theatres in that era did not have THX quality sound systems. They had a lot better speakers than your TV did (and certainly better than the little drive-in speakers you clipped to your car window) but they were far from spectacular.
Date night in the sixties at the drive in was sometimes like Meatloaf’s
Paradise by the Dashboard Lights
Later, I would back my pick up into the front row and set up lawn chairs in the back. With a cooler full of beer on ice and food we brought in it was a great way to spend a summer night. The management did not seem to care much about it except the night I set up a charcoal grill and was getting ready to grill some steaks.
One drive in had all night specials sometimes. I remember a Planet of the Apes marathon that lasted until dawn. Also a John Wayne marathon. They are all long gone here. The last one went when a country club bought all the land and built a bunch of multi million dollar houses around a golf course.
Two films have important scenes at the drive-in that immediately come to mind are Grease (where “The Blob” is on) and The Outsiders. In the latter I think all we see in the movie marquee at least is “The Nightly Double Feature”, Dallas (Matt Dillon), Pony-boy (Howell) and Soda-pop (Macchio) sneaking in and sitting in outdoor seats near the concession stand. The first film I cannot recall - some movie about a sea-rescue while Dallas is hitting on Cherry (Diane Lane) - then some Buster Keaton (?) shorts on-screen while the in-film action occurs.
Both films of course have lots of cool cars - many of which are convertibles so no probs seeing the screen.
Films that me and friends saw at the drive-in that come to mind are the original (Michael) Keaton Batman, Rumble Fish (no drive in scene!), Young Guns and the Dinosaur Documentaries Spielberg made in the 90’s.
The movie that comes to mind for me is Twister, when the tornado tears apart the drive-in screen and everybody cowers in the concession stand walk-in.
So I found the entire theatrical cut (which I may not have seen - on archive dot org) of Coppola’s The Outsiders and a wide shot shows the marquee:
BEACH BLANKET BINGO
MUSCLE BEA H PARTY
(the C is missing - probably stolen by Pony Boy (C. Thomas Howell)
Two of the 7 (!) Frankie Avalon / Annette Funicello beach films - the first with Buster Keaton and the latter with (little) Stevie Wonder.
We go to the drive-in at least once a year. It’s a summer experience we all look forward to. We back the SUV into the space, make the trunk comfy for the little ones with blankets and pillows. I have a Bluetooth hooked up to an AM/FM receiver for the sound, and we sit in lawn chairs.
We bring some food, but we make a point to spend plenty of money at the concession. Since admission is per car, they make most of their money off of food. It (like probably all drive-ins) is a family run business, and I’m happy to support them.
There’s some third-rate horror film from the '70s where one of the victims is stuffed inside a soda machine at the DI and hooked up to the dispensers and when you get your drink, it’s a cup of blood!
I don’t care enough to go look up the title.
One of my most memorable drive-in experiences was going to see Halloween with my stepmom. This would have been, oh, 1978-ish. My Dad wasn’t into the drive-in (I recall him taking the family only once when I was a child), so my stepmom and I went on our own.
Naturally we hit the store first and bought every imaginable sort of junk food. Then we got to the venue early so we could get a prime parking spot.
Holeee sheeiiitt!! For its time, that was the scariest movie going. And it did the trick. We were scared out of our wits!! All the junk food was forgotten as we sat with saucer eyes and glued to our seats. I remember I locked the car doors.
As soon as the movie was over, we hightailed it out of there for the safety of home. Just as we pulled into the driveway, a station wagon with an emblem on the side cruised slowly past.
I don’t think our feet touched the ground as we shrieked and raced for the house.
One I know of in the Sacramento area.
The most memorable and one of the earliest film scenes set in a drive-in is in Peter Bogdanovich‘s first film, Targets.
Also, in Twister, they are showing The Shining at a drive-in, and a tornado rips the screen away and you see evil Jack Nicholson’s face - you know which scene I mean - projected on the twister. Inspired!
Triple feature: The Conqueror Worm, Torture Garden, She-Devils on Wheels. That was one of my formative teenage drive-in experiences.
How come I remember this so well? Because it was so different from watching TV, or a movie at a regular indoor theater. Something about the situation just makes it stand out, even minus alcohol and making out.
The Mendon (MA) Drive-In has a deluxe snack bar. If only it were closer to Boston.