All right, it seems that you just want to know why they choose to wear sunglasses in that way. As far as I know, baseball players always face the problem of strong sunshine, which could make their vision worse and dizzy, so they need to protect their eyes. Sometimes, they just want to look cool and better. But most importantly, sunglasses are used to block sunshine and make them focus on the game.
A lot of baseball players wear sunglasses upside down to make them look cool. They could protect their eyes from the uv rays in the strong sun lights. They could also see clearly through the special coating of the sunglasses in the games. That is to say, the sunglasses will make them play better in games.