I know, I know, this sounds like a ridiculous question, but bare with me.
As an example of my lack of understanding, let's take the advanced variation of the French defense.
- e4 e6
- d4 d5
The opening books say something like ' white advances his e-pawn to 'gain space in the center'. Now, try as I might, I simply can't see where the extra space is that white has gained!
He can't use the f6 square or the e6 square as these are covered by pawns, knight and bishop. He cant use the e4 square as that is covered by black's e5 pawn. So where on earth is the 'extra space' that white is supposed to have gained?