Redirecting the knight to a5 where it will eye the c6-pawn and help White to fight for the open a-file.
Now the knight isn't just blocking the d-pawn, it also attacks the weaknesses on c4 and e4.
1...Nd5? 2.Nxd5 exd5 would be a strategical error, since the d5-square could no longer be used by the pieces.