People play no-limit hold'em for all sorts of reasons — the fun of competition, the entertainment value the game provides, the opportunities for social interaction, and more. Of course, winning money is the central motivation for serious no-limit hold'em players, and it's usually a priority even for amateurs or "recreational" players as well.
If winning money is important to you and one of the reasons why you play hold'em, understanding expected value is of primary importance.
Understanding Expected Value: Defining EV
There are a couple of ways to think about the concept of "expected value" or "EV" for short.