In life and poker the more sound information you possess, the better quality your decision should be.
When you act first, you don’t obtain the information from your opponent’s actions like you do when they act beforehand.

When your opponents check or bet, they provide information regarding their hand.
A good hand-reader should be able to narrow his opponents’ range based on their actions.

If your opponent checked, does he check strong hands or only weak ones?
If he bet, what was his bet size and how does that define his hand?
Did your opponent display any tells?
The questions are endless.

All this information increases your decision powers when you act after your opponents.
This is why you should play stronger hands in early position and can play weaker ones in late position. Additionally, raising in late position as a tactic to fold those yet to act adds additional value to your raise because you won’t have as many players behind you on future streets. Furthermore, when you already hold the button, raising loses some of its value because you already have the best position. When attempting a “button stealing raise,” always also take into account the propensity of the players behind you to fold.

Of additional importance is your position in relation to a bettor.
If a player bets and you’re next to act, having more players to act after you increases your risk and lowers your information-gathering possibilities. It’s much better to “close out the action,” have the bettor lead into your other opponents and then be last to act. This concept needs to be added into the equation of whether to bet or not.

Sometimes you can maneuver yourself into this position.
A good example of “closing the action” is semi-bluff raising out of the BB pre-flop.
You’ve seen the action of all your opponents, and if none of them have indicated strength, the “fold equity” of your raise is higher.

Many players, particularly inexperienced ones, undervalue position. Having the extra information available can make or save you many bets, often bets with large value, particularly in big-bet games. Plus, advantageous position can generate more profitable bluffing situations, winning you the whole pot.
So, weigh your position in your poker decisions. When you’re assuming greater risk because you don’t know what your opponents will do, tighten up. Conversely, when you know your decisions are going to be made with a high level of information, you can play a little looser.

Play Well and Good Luck!