why Nf6 is played against d4??


To challenge the idea that white can follow d4 with e4 and build a big center. That is also the point of d5 as the response to 1.d4 as well. with either 1...Nf6 or 1...d5, e4 pawn push is not immediately possible. 3 Nc3 puts the idea of pushing e4 back on the table, by introducing a defender of the e4 square.