Another potential reason for Sosa using the corked bat in BP was to build confidence. Basically saying hitting monster homers in BP would carry over to the game. I think I heard it from one of the Baseball Tonight guys, but I’m not totally sure. Either way, I don’t really buy it, but I guess it kind of makes a little sense.
As far as the question about BP being for practice or not, I agree with what Nametag said. It definitely isn’t practice in the sense like you are going to learn to hit there, but on the other hand it isn’t for fun or really for show either. Well, it is a little bit for fun a times, and maybe even a little more for the fans at times, but in general it is for the players benefit to help them in the game. It is mostly a warmup.