Raising to cut down how many players stay in the pot.
The more players, the more likely someone draws to a strong hand, so a hand that is ahead now, like one pair, wins to the end more often against fewer opponents. You hear it most in Limit games. Example. In Limit Hold'em with top pair, when a bet comes to you on the flop, raise instead of calling so the players behind fold. Top pair holds up more often against 1 opponent than against 3.