Batteries multiply force along a single line. A doubled-rook battery on an open file can overwhelm a defended square; a queen-and-bishop battery aimed at h7 (or h2) is the engine behind many kingside attacks. The piece in front delivers the first blow while the piece behind recaptures or renews the threat.
Building a battery is a standard way to convert pressure into a breakthrough. The defender often has to give up material or allow a damaging exchange just to break the alignment, so creating and aiming batteries at the opponent's weakest point is a core attacking technique.