It may seem as though White and Black start with equal forces, but this is not completely true. White, who moves first, has the advantage of being able to control the center and develop a piece one move before Black. In the hands of a skillful player, the advantage of the first move will endure well beyond the opening.
A good first move puts immediate pressure on Black. The biggest drawback of the A00 moves is their failure to apply any pressure. Playing a move like 1.a3 says, 'Pass! I'd rather you move first.'
A move like 1.d4 says, 'I'm playing to win by occupying the center and by developing my pieces quickly.' In fact, if White could play again without allowing an intervening move by Black, 2.e4 would produce the position shown in the diagram.
Here White has such a powerful initiative, that Black might already be theoretically lost. That's why the best responses to 1.d4 prevent 2.e4.
The position after 1.e4 is similar, but different. The best responses to 1.e4 can't prevent 2.d4, which White can play after any first move by Black. They can, however, neutralize it.
