If it's competitive game you are in there to play for the "win" and fun comes from trying to win for it. ie. if you play a fighting game in none ranked queue to try a new character, you learn nothing while not trying to "win" thus even not-MMR-ranked queue will usually have a hidden one stick to it so you don't play against players that are too good/bad against you. (yeah, I know the practice landing hit-confim or combo/setups against real human player is a thing and not thinking about winning. BUT the end goal is "winning" with your new tool. )

ranked games are the trying to play at higher level mode where you try to push your skill up and thus can play against higher tier players. And to my opinion, it is also fun and why people grind.