It's about taking players off their game. It's about making them play your game. Making players play your game is both complex and sometimes awfully simple. While this play also happens to be one of the most often mangled, backfiring plays in all of poker, when it works, it's a beautiful conception. For one unit of betting you get to see two cards, rather than seeing those two cards for two units of betting, and if you do happen to make your hand, the pot has more money in it already.
Defense comes in many other forms too.
You can change the tempo of a game to suit your preference, or un-suit the preference of a key player at the table. You can make a raise in position with a normally non-raising, speculative hand because your opponents may likely check the flop to you, giving you a free card again. And of course, you can fold rather than mix it up in a pot. As much as possible, a winning player should be sitting in games most suited to his or her style. Sometimes all it takes is making a sacrifice, intentionally losing bets or pots, to turn a sleepy, dead game into a real barnburner.