Why Nf1 is best in this position?

Other than giving up your bishop pair, ...Be4 (followed by white's capture with his knight) means you have to take back on e4 with either the rook or d-pawn.

Taking with rook will give you an awkwardly placed piece, and taking with pawn gives you an isolated pawn and lets white push his d-pawn.

So it's an inaccuracy, not blunder.

Could you edit your post and resubmit, this is a fix to the fact that the board is not showing up.

You still  cannot see the board?

I can see it now. I think the idea here is Nf1 is the fastest way to get the inactive knight back in the game, with Nf8-g6-f4, since the knight isn't doing anything on d7.

There's something more  I think about the move Be4

After(as the game continued) Ne4 Re4 Bd3 Re6 Re1,white can change an inactive rook with black e-file rook

In the original postion before Be4, white pieces are not "coordinating", my minor  pieces and e8 rrook are more active than my opponent's . I should activate d7 knight and a8 rook ,other than change pieces.