You must find safe places for your pieces. The easiest way to do so is to push a centre pawn.
Example: if you play 1.e4 (and follow with d3 or d4 later on), you can develop your knights to f3 and c3/d2, one Bishop can go to c4 or b5, the other can go to e3 or g5, and your pieces are relatively safe from being driven away to bad squares.
If you don't push a centre pawn early on there is the danger that your pieces get stuck behind your pawns.
I was wondering about opening principles: push central pawns, control the center, develop pieces , castle fast. I can understan developing, castle (to save the king), and control the center (pieces must be in the center because in the center they are more active, so control the center is important), but what about pushing central pawns? Only because you can develop bishop and queen or there is another reason? What do you think of opening principles? They tell me them, but never explained.