…like these. What are the strings supposed to be for?
It looks like those are drawstrings to tighten the bottom of each leg of the shorts against the wearer’s leg. I guess that would be to stop or at least deter unwanted guests from crawling up into and among one’s privates. I’ll leave it up to your imagination to picture what or who those visitors might be.
That’s all I got.
I think they run inside the hem to tighten them. Who knows why anyone needs snug hems though.
ETA “unwanted guests.” Makes sense.
As other said, they appear to just be nothing more than draw strings for the bottom of the legs. Given that it would probably look kinda stupid* (IMO) to pull that tight, I’m sure it’s just for looks. I have a co-worker that wears nearly those exact shorts, they look just fine with the strings hanging down. More or less like this. It’s (again, IMO) like asking what the epaulettes on (off the rack) coats are for. Sure, they have actual uses, but if you buy a regular coat, for day to day use from a regular store, it’s just there for looks (and to get constantly unsnapped by your seatbelt).
*To cover my self, there was one picture that made me slightly unclear as to whether the drawstring was to tighten the shorts around the bottom of the leg or to ‘hike’ them up a bit (shorten them, if you will) to give them a different look. However, I think that was just the look those specific shorts had and what I said is still the case…that they’re draw stings and really just for looks.
Oh come on… They are obviously intended for the guy who had a dodgy curry the evening before and is unsure whether he can hold on long enough.
Actually, this makes sense, especially for long-ish shorts. If you are wading across a stream or something, you might want to hike up the hems and pull them tight to keep them dry.
Except for the 99% of the people that buy them that will never, ever, ever set foot in a stream.
Well, yeah, but the people *looking *at them will be so admiring and jealous of them for their extreme outdoorsy-ness, amirite?
If you’re only worried about the bottom of your shorts getting wet, it’s not really that much of a problem.
I have a few pairs of capris (short pants) that have those on the bottom. They kept getting untied so I just double-knotted them into bows. Not very stylish in any sense, but they were comfy and I don’t care much anymore. 
In this case I think they were meant to make the capris more shorts-length, but who knows?
Clearly they’re patterned after western quick-draw holsters. You tie them around your legs so the shorts don’t move around and waste your motion when you’re rapidly whipping your cellphone out of the pocket.
They’re for any surprise 18th century reenactments you might encounter. Turns those shorts into respectable breeches toot sweet. Or your capris into plus-fours. Just fold your sun hat into a tricorn and you’re as handsome as Tom Paine!
Oh, shorts. Misread title (honestly), was looking forward to ancillary pronunciation duels: Tzitzit - Wikipedia