For me, I use Metacritic to help me decide what not to buy, rather than what to buy, which I think is kind of unfortunate and backwards, but it's how I use it.
Looking into a game, I usually know before hand whether I am interested in the game or not. I am generally very interested in JRPGs, whereas I have absolutely no interest in first-person shooters or hack n' slash games. Then, if the game score highly enough (the score needed for me to buy a game usually fluctuates based on the cost of the game, but I will say that I am generally very picky), I will be more inclined to buy the game. Naturally, the higher the score, the more inclined I am to buy the game.
Again, I do wish I used Metacritic in a different way. I feel like there's too much of an emphasis on scoring games harshly so that they can be ranked, rather than scoring games more generously so that people can broaden their horizons and play more types of games in the genres they enjoy.
I understand that the last two paragraphs are kind of contradictory, so allow me to explain myself. When Bayonetta and God of War get scores in the 90s, that's fantastic, because even though I don't like either of those games because I don't enjoy the genre, people that do enjoy the genre should know that those games are great. On the other hand, take the Dangan Ronpa games. While their Metacritic scores are respectable, they're not fantastic. Sure, you could make the argument that the gameplay is weak, but are there really any games that do what it does close to as well that particular game does? Here, I feel like Metacritic is ranking the game compared to other games arbitrarily, rather than encouraging you to buy games that are only really appreciated by its ardent fan base.
But let's all face it: the only real purpose of Metacritic is to compare arbitrary numbers so that our platform of choice looks better when we defend it on internet forums.