Good position is a primary source of value for many poker hands. It's often more important than the cards themselves are. Aggression is how you realize value in poker, but position and the use of position in guiding your aggression is the source of the value.
Your position determines how much information is available to you when it's time to make a decision. Assuming you can evaluate and use it correctly, the more information you have the better decision you will make. Also, the less information your opponents have about your hand the more likely it is they'll make a bad decision. Position controls both sides of that coin -- how much information you have and how little information they have.
The value associated with position is easily demonstrated by looking at hold'em or Omaha -- games where position remains fixed for each betting round. In those games, if you're in late position every player (with the exception of the blinds in the first betting round) has to act before you do each betting round. Throughout the play of th ehand you'll have the maximum information about other's hands while they'll have to act with a minimum level of information about your hand.
If you're holding a hand like A8 clubs it's a lot easier to bet into a field of four players with a flop of K84 and one club if they've all checked than it is if you're first to act. That's not to say you shouldn't be betting that hand from early position, but it is a lot more iffy than it is from late position after everyone has checked.
This material appears in a slightly different form in my book The Complete Book Of Casino Poker