Raising early in the hand helps to set up a later bluff. If a player raises pre-flop and also after, then the bluff can be carried out by a significant bet after the turn card and the river card. It takes guts to do this, but a bluff only works by making bets that are larger. A small wager will not run off a player with a good hand.

You need to get the other player to imagine what you hold that will beat their hand. Three cards to a flush in the flop is an ideal place to make such a bluff.