10.Ne2

Why not develop the King's Knight on f3? This is, after all, the natural square for the piece.

One reason is strategical. White is still considering the future placement of the other Knight and decides that it would be equally well placed on f3 by Nb1-d2-f3. After White castles O-O, the King's Knight can take up an active post on f4 or g3, depending on circumstances.

Another reason is tactical. After 10.Nf3 Ng6, Black is simultaneously attacking the Queen and threatening a check on the e-file. White could avoid the worst with 11.Bxg6, but wants to keep the light squared Bishop. Otherwise its opposite number would rule the b1-h7 diagonal.

Next move: 10...Ng6