Defend Queen's Gambit

If you don't know what the Queen's gambit is, it is a offer of a pawn on the queen-side. But is really a gambit? Qa4+ instantly gets the pawn back. It also gives a good central position. Black has no developed pieces that are useful. The only problem about this for white is that it develops the queen too early and also it breaks the principle of only move a piece once in a opening. I still think it is worth it.