What would happen if you were to remove XP from a fantasy game? This isn't solely a question about traditional games like D&D since TSOY too had XP.
If you have to define XP to clarify your statement, go ahead. I assume that XP, in the end, increases the power/effectiveness of the character. Even XP spent in TSOY to buy a new Key presumably does so because the player wants to move the character to that mode of play and thus get more XP.