[mrob]

One of the most important concepts in Go is the idea of initiative and forcing moves (see "sente" and "gote"). Making an unnecessary capture is giving your opponent initiative, so it's a "gote" play. They're free to play anywhere else on the board, which is bad for you.

Sometimes a group of stones is in a position such that no matter how their owner plays there is no way they can be saved (assuming correct play from the opponent). The stones are called "dead". Instead of capturing those stones, it's better to play elsewhere and retain the initiative ("sente"). You retain control of the game and your opponent is forced to react. You're not losing anything, because you can come back and capture the stones any time. If your opponent tries to save them you know a move that will prevent it.

Recognizing stones that can't possibly escape being captured, and knowing how to stop them escaping, is the study of "life and death". The better you get at this the less likely you are to waste moves or give away the initiative.