Shortly put, the more information you have the more you know where you stand with your hand.. Also position allows you to steal a lot more pots and it also allows you to not commit any more chips without seeing the action infront of you.. Based on what your opponent's betting patterns and habits are, you can make deductions and use your position to win the pot rather than the cards you have.. also you can lose less when someone seems strong and the move coordinates with his previous betting pattern..

Next to observation, I think it's the most important thing in Hold'em

Of Course, there is an advantage to being out of position and that's being the first person to be able to bet at the pot.. if you're in position and have nothing, it's tough to call a bet from someone who is out of position and made a good bet (even though he might have nothing)