I'm somewhat of a Chess newbie so please excuse me if this sounds ignorant.
I understand the concept of having a material advantage but if I take an opponent's piece that isn't yet in play, how much of an advantage is that?
Like if I take a rook locked up behind the bishop and pawns that couldn't even come out for several moves anywayn.
It depends on the position. If he's using his other pieces to make a mating attack on you, or force an unstoppable passed pawn, you probably don't have much of an advantage! On the other hand, if the position is fairly quiet, and/or you can easily force trades, your material advantage will become practical in the long run, once you simplify to an endgame and you have a rook and he doesn't.
You bring up an interesting point, I do know some players, some of them pretty good, who deliberately sacrefice the exchange (rook for minor peice), because they beleive rooks are useless in the begining and their minor peice is better.
This is a new kid on the block strategy, that is filled with flaws. I love it when players do this kinda sacrefice! I just open up a few center files early, and bam, that "useless" rook is a killer to them, killer!