The NFL's preseason games are set to start on August 8. It warrants the same reaction every year, wherein people start talking about the start of preseason and then say "It's preseason, so it doesn't count."
But they are wrong. The preseason has a purpose. It may not count toward the win/loss total, and the starters may not play the entire game, but it does count.
If we had no preseason, teams would have to look at the players they want to start during the regular season, taking time away from the games and forcing teams with rookie pickups to lose valuable experience and a opportunity to make a huge play on the ball.
If we had no preseason, the rookies would not ever get a chance to play their first season, because nobody would know what they could do and nobody would want to take a chance on someone unproven, especially given the fact that the regular season only has 16 games (for now) and every game counts.
Another reason for the preseason is to get the players used to having a game every week and the physical and mental requirements of that. The players are also exposed to the feel of what they can and can't do.
While the risk for injury is always there, teams and some players like the preseason because the whole schedule and the roster as far as starters are not usually complete. Seeing unproven players in these four games gives coaches a chance to analyze statistics, see if rookies are worth a starting spot, and see if veterans should hold on to their position.
The preseason is also good for the NFL because, after a few months, the players get antsy and want to play a football game. The fans get antsy too and want to watch.
Most people say that the preseason doesn't dictate the rest of the season, and to some extent that is true. The Detroit Lions had an amazing preseason—undefeated, in fact—and did not win a single game in the regular season.
Preseason games can also be considered extra practice. Limas Sweed of the Steelers might see it that way because of his "well-known" dropped pass during the AFC championship game against the Ravens. The games are a good chance to learn the playbook.
So while it may not seem like the preseason matters, in fact they play a huge role in the attitudes, positions, and shaping of an NFL team.