This move is a novelty. The knight needs to step out of the pawn's way for the black bishop, which has also done its job and seeks a better diagonal.
1. e2e4  1... e7e5  2. g1f3  2... b8c6  3. f1c4  3... f8c5  4. c2c3  4... g8f6  5. d2d4  5... e5d4  6. e4e5 Probably the most fashionable Italian line nowadays. 6... d7d5  7. c4b5  7... f6e4  8. c3d4  8... c5b6  9. b1c3  9... e8g8  10. c1e3  10... f7f6  11. e5f6  11... e4c3  12. b2c3  12... d8f6  13. e1g1  13... c8g4 And this pin is still there. 14. b5e2  14... g4f3 This is the second most popular choice, although Black is not forced to part with his bishop pair. 15. e2f3  15... a8d8  16. f3e2 The bishop has done its job, and now it is time to look for a better diagonal.