Levelling up in a game gives you advancement in the game , this give you progression to aim for a higher objective from your previous task.
Levels can mean many different mechanic .
Levelling that shows other people your rank in game.
Game levels when you accomplish a level you move onto the next one.
levelling up is one of the biggest mechanic in RPG and MMO games, this gives the player something to aim for and to accomplish on reaching the highest level.