These are two reasons I would have given. If the lock on my violin case worked, I’d be able to leave things in there which I end up keeping in my pockets on stage, which is a real pain. And I’ve got pupils who’ve gone out and bought lockable cases specifically because too many others think they can just ‘borrow’ an instrument, because, y’know, they’ll look after it :rolleyes:
::Quitely chuckling in a smug string-player way at the idea a $1600 instrument is something even worth worrying about:: 
Remember when you could lock your luggage, then hand it over to the airport baggage people? I suppose if you entrusted your guitar to them, same deal. They probably wouldn’t be bold enough to steal the whole thing, but they might like to rifle through it for a tuner, capo, or whatever else you might have in there.
Of course now you can’t do that. But there may still be times, like if you were having Mayflower move you, where you temporarily entrust it to someone else. Mostly now it’s an added security to keep from dumping your instrument on the ground as you carry it.
Guitarist here, and I agree with the keep the *rugrats/other players * from messing with it line of thinking.
My guitar came off an airplane once, with several of the latches opened, but the lock still in place. Just a standard combo lock, but did the trick. Don’t know if it stopped a thief or just a curious baggage handler. Either one would have been disappointed to find the Japanese knock off inside of the Gibson case. 
For some reason I imagined your guitar falling out of a plane in transit and trying to figure out how the hell you noticed that before you noticed your guitar was plummiting at terminal velocity. Then I attempted to figure out how you got it back. Then my brain kicked in.
So basically, we can say the locks are there to keep honest people honest. Kind of like the latch and padlock I installed on the freezer I keep on the porch outside after somebody got into it and stole an entire case of my vegetarian roommate’s veggie burgers (we joked afterward about what the thief must have said when he got the stuff home in the light and saw what he had). The latch and lock would be a piece of cake for a determined thief to break off, but it’s enough to discourage the opportunist who sees the freezer and wants to “see what’s inside”.