If we leave realism aside for a moment, though, the difference is that:

• a carry slot limit restricts how many distinct kinds of items you can have, whereas
• a carry weight limit moreso forces you to leave behind duplicates of items or to e.g. make a choice between heavy armor vs. having auxiliary items.

It depends on the game what's more fitting for the overall design, but yeah, ultimately you want to prevent the player from optimizing the fun out of the game.
Having very heavy armor or dozens of healing potions can be boring, since there's no risk anymore. But having one of each different kind of item can also kill the fun, because having the perfect solution for every situation is just as boring.

Sometimes I would appreciate a "Loot all except junk" button, and possibly having the option of manually marking certain items as junk

Remember that brief period where games would have items specifically designed to be vendor trash?

Okay, but make crafting mats 0 at least or have a second "hoard" inventory that doesn't count but is also inaccessible in combat or something. Just give us a way to clean out a dungeon without having to do a bunch of time wasting bullshit.

All the junk in fallout? I know it's worthless, but my compulsion is to loot everything.

I'm not opposed to those that want that mechanic, but it would be nice for there to simply be a toggle to disable it.

I can't stand going into a dungeon and on the third chest having to decide what to throw away this time.

I'm ALWAYS going to have a max inventory, it's stupid not to play a game without it. If I can carry 10 healing items, I will just in case. But it also means at every single drop, I have to go into my inventory and decide what to toss. Or if to toss. It's takes up time when want to do the fun things in the game. And it makes me have to constantly make decisions and think if this item is better than one I have, and if so what do I get rid of. Do I get rid of healing potions or mana potions? When is the next time I can sell this? How much is this worth?

I play games to play games. Not to micromanage inventory.

Just because a bad mechanic has a purpose doesn't mean it's not bad.

I hate being forced to think about that shit.

I love a partial solution in Cyberpunk where healing items simply have a cool down.

Odenst make lore sense really, but you can flavour text that up.

Weightlimits mostly serve to add annoying inventory management to a game that is supposed to be an escape.

They should design their games better to not need the player to do that or horde things.

Its part of why I hate how "loot" is typically done.

Better save this potion for later, I might need it.

I don't think anyone likes Fallout 76

I played until they patches the bug where if you equipped/unequipped gear that affected carry weight fast enough it would't remove the effect every time and you could max it out.

Also, I hated that your stash was also limited by carry weight.