When you’re in a poker hand your odds to win will always be better if the number of players is smaller. It’s always better to be head-to-head against an opponent. It’s not always possible to be head-to-head, but there are tactics you can use to try and attain that. Isolating opponents is not only a way to improve your chances in a hand; it’s often a method of taking advantage of some playing styles you’ll run into out there.
A common situation where you would try to isolate an opponent would be when a small stack has made a big move with an all-in bet, or pot-committing raise. If you want to try your chances against this player, and there are other players to act after you, you can try to isolate that player by raising it up even more. This will usually get the other players out of the hand and increase your chances of winning, while not risking any more chips if everyone else folds. The strength of your hand should determine how aggressive your raise is, since there are still players to act. This move works best if the short-stack raiser is in a late position, and the other players in the hand have acted once in the hand already.
Another time you’ll try to isolate an opponent is when you’re playing against a maniac, and you’ve been dealt a premium hand. Once the manic raises pre-flop you can re-raise and get the other players out. Then you’ll be in a situation with an aggressive player who most likely has a mediocre-to-weak hand, while you’re holding a premium hand. This is one of the most profitable situations in poker, and by far the best way to play against a bully.