Thanks for the replies so far, it does sound like they are used for convenience most of the time. As for why I haven't used them for convenience, even though I have them, they have never been with me when I am doing car repair. I live in an apartment, have to take my tools to a friends house to do any work, and I've been just taking shallow sockets with me, after watching videos and not seeing the need for a deep on any fasteners I have worked on in my cars.
I do see if I had a stocked toolbox next to me, I would grab the deeps instead of shallow as long as the deep can fit, even if it's not needed. That does make sense.
What I have decided to do is put together two mobile kits of sockets. I'm going to keep an 8mm to 19mm shallow socket set in 3/8" drive on a rail in my car trunk toolkit at all times. I'll add in a Quinn 3/8 ratchet next coupon. That's my emergency kit. I've already put the socket rail together, and I added in a few SAE sizes that dont overlap with metric. Just in case I come across something SAE, not necessarily car related, this is in my car trunk tool kit. Added in 3/8", 1/2", 9/16", and 11/16" in SAE sizes. Also going to consider a combination wrench set in the same sizes, once I decide on a storage system (leaning towards a wrench roll for this)
Then, separately, I'll have a car repair specific kit, that I dont keep in my car at all times. But is pre-packed and ready to go. It's 3/8" drive shallow and deep in 8, 10, 12, 14, 17, and 19mm. Those are the sizes my car uses. There's also 22 and 24 and 27 under the car but if I start doing drive train stuff, I will bring the 1/2" impacts plus breaker bar, impact wrench, and ratchet.
I'm also working on some ratcheting wrenches and possibly XL 0 degree fixed end wrenches, just in the sizes for my car, an 8-10, and a 12-14, might not even bother with 17-19 since I haven't even hit one of those sizes yet in any of my work, I know they are there though.
I'm not interesting in hauling a full set of shallow and deep everything to a friends house to do car work, but if I have shallow and deep for my JIS sizes, and then I have a full shallow set for just in case I install an aftermarket part with a non-standard size, then I think I'm covered.
Sorry to those who think I'm trolling, I'm not.