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Thinking along the lines of this question, and the too awesome to use trope in general. Why do game designers want to include consumable items in their games?

Mortal Kombat 11 got me thinking... Why include consumables that give you an edge in battle at all?

(I’m looking for concrete answers for what it is that consumables add to gameplay)

Well, in a roguelike or something, where eventually if you don't use your consumables they expire (because you die, or win), then consumables provide another layer of medium- to long-term strategic planning for the players to think about. Instead of just making sure you use your renewable resources effectively in each encounter, you now can also consider spending consumables per encounter at a sustainable rate, but not too few that you die (and lose all your items).

For some games, consumables are a way to keep players engaged (monetarily-speaking) by letting players buy the same bonus over and over again. Instead of having to keep adding power creep upgrades for your most paying customers to buy (and thus making it so the non-paying players don't even think they have a chance because they're so far behind), you can sell them the same item repeatedly, every time the previous copy they purchased expires.

In addition to what Foxwarrior said, consumables are a perfect way to include effects that would break your game if they were available all the time. Imagine you make a game level that is exactly balanced with your player abilities, and then some player gains an unlimited healing capability.
The balance is right out the window, the level will be too easy, you will have to introduce much harder levels afterwards to account for that healing capability. Worse, in a group of players one will be assigned the boring task of a "Healer".

Give the player a single potion of healing he can consume - the balance stays where it is. It's just a nice one time bonus for emergencies.