I know, I know, this sounds like a ridiculous question, but bear with me.
As an example of my lack of understanding, let's take the advanced variation of the French defense.
[title "Advanced variation of the French Defense"] [fen ""] 1.e4 e6 2.d4 d5 3.e5
The opening books say something like "white advances their 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!
They can't use the
f6 square or the
e6 square as these are covered by pawns, knight and bishop. They can't 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?