The knight is getting back  into play. More importantly, it hits the c5 pawn, thus preventing the  freeing d6-d5 advance.
1. c2c4  1... c7c5  2. g2g3  2... g7g6  3. f1g2  3... f8g7  4. b1c3  4... b8c6  5. g1f3  5... d7d6  6. e1g1  6... c8f5  7. b2b3  7... g8f6  8. c1b2  8... f6e4 A logical novelty. Black makes use of the pin  to trade the dark-squared bishops. 9. c3a4  9... g7b2  10. a4b2  10... d8d7  11. f3h4 Topalov on his turn uses the shaky  position of the black knight to secure the dominance of his bishop. 11... e4f6  12. h4f5  12... d7f5  13. e2e3  13... e8g8 