Consumables can be a way for your player to pass difficulty spikes in your game.
Common game design wisdom is to create a gradually increasing difficulty curve. But when your game is complex and its pacing more driven by narrative than gameplay concerns, then this is easier said than done. So you will often end up with situations which are suddenly far above the difficulty curve.
Imagine your player facing such a challenge at one point of your game and they can not overcome it. They are simply not skilled enough and they don't have the patience to train. This could be the point where the player abandons your game in frustration. Or it could be the point where the player looks through their inventory, consumes every consumable they have which gives them an advantage, passes the challenge and continues with the game.
So consumables are one design element which adds dynamic difficulty to your game. When the player is not challenged enough, they will feel inclined to hoard consumables and in turn make the game more difficult for themselves. When the player feels overwhelmed, they will use their consumables to bring the difficulty down.
It is one of the few ways to create dynamic difficulty which is both under the control of the player (unlike automatically making enemies weaker when the player loses) and plausible within the fiction of the game (unlike a difficulty setting in the game options).