The basic idea of the opening was to snatch
e5 pawn, but by playing
Qg5 Black returns the pawn with good play.
This forced White to seek other ways of reaching advantage, so he tried with
Bxc6 counting on a won pawn endgame. However, things are not so easy since Black has ample of compensation in his bishop pair-remember that bishops are good in open positions-and doubled pawns control a lot of important squares.
Since that try failed, White needed to try something else, and the only option left is bishop retreat. If they play
Bc4 Black can transpose into Italian game with the extra temp since
a6 has been played. Since
a6 is an important move in those lines for Black, he would gain a small plus from the bishop retreat. That leaves
With this retreat White keeps pressure on
e5 pawn and hopes to force Black into making some concession. Black will do his best to develop and defend without weakening himself but in the end he fails to do so. He must play
b5 at some point to release the pressure from the White light-squared bishop. After
b5 Black will have compromised pawn structure on the queen side ( backward
c pawn and exposed
b5 pawn ) and that is the concession White hoped for.
It isn't much-Black is still OK-but it will guarantee White a long initiative-which is better overall result than what other lines offer.
The reason for this being Black's
c-pawn is blocked by a knight so he can't participate in the battle for the center. This enables White to seize initiative in the center by forming
e4 pawn center supported with the
c3 pawn. Black will lose time to reorganize his queen side in order to counterstrike which gives White time to meet comfortably Black's plan. This usually involves
Nbd2-Nf1-Ne3/Ng3 maneuver, preparing king side attack, but can be
a4 thrust, or simply a favorable change of the pawn center's type-he can block the position with
d5 or open it with an exchange like
The last option is the most dangerous for Black so he needs to play very passively for a while before he counter-strikes.
Wikipedia link you can see the list of variations. Notice that in the main line there is one thing in common for all the Black's responses-they regroup their queen side to meet White's initiative.
By retreating the bishop to
a4 White gains small concession from Black in view of compromised pawn structure which results in White's better control of the center, which results in small but lasting initiative.
Hopefully this answer sheds some light for you.
If you have further questions leave a comment.