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