hayton3: This game is featured in Silman's "Reassess your Chess".
He examines the postion after White's 20th move. The position is clearly better for White with his bishops pointing ominously at the Black kingside. He also has more central space and the opportunity to attack the black pawn on a5 more times than it can be defended.
In light of the above Alekhine decides, quite rightly, to do something active and plays 20...Rb4! with the threat of Rfb8 and the ensuing domination of the b file with possibilities for active counterplay. Consequently his opponent accepts the the exchange sacrifice with 21.Bxb4 and Alekhine recaptures with the c pawn 21...cxb4!.
Thus Alekhine improves his position by:-
1) Making White's c pawn backward and a target.
2) Creating a beautiful outpost for his knight at c5.
3) Establishing a passed pawn which could be a deciding factor later in the game.
4) Joining the c pawn with the a pawn to elimninate the weakness on a5.
All these advantages far outweigh the sacrifice of the exchange and highlight some important ideas in positional sacrifices.