There are 2 points:
- On e7 the knight can go to either g6 or (after ...c5) to c6. On f6 it can only go to h5, loses a pawn to Nxe5 and Qxh5 or to d7, blocking the diagonal of the important Bc8.
- On e7 it allows the move ...f6 to support e5. This is most clear in the Derdl variation 1 e4 e5 2 Nf3 Nc6 3 Bb5 a6 4 Ba4 Nf6 5 O-O Be7 6 Bxc6 dxc6. White deliberately loses a tempo Bb5-Ba4-Bxc6, because Nf6 and Be7 are misplaced: should be Bd6 and f6.
I like to play the c or f pawns and then advance the knight behind (both black and white) at certain times. Here I don't see the point, and looking at a Stockfish line following it through, I still don't see the point in Ne7. What would you have done in this early opening position? Nf6, Ne7, something else?