Black’s pieces are compressed, and it might appear that his bishop on e7 mostly is helping to protect the pawn on d6 and knight on f6. But it is doing more: it aims through the pawn at the White bishop on a3, which is loose. After the d6 pawn marches forward to attack White’s knight on e4, Black can play BxB. But the board isn't ready for this yet; White would be able to ignore the pawn and play BxB himself. After Black plays the recapture NxB, White can move his e4 knight away with no net loss. Black’s problem is that his pawn needs a better target—one that requires immediate attention and thus creates time for him to be the one to play BxB a move later. The natural way to achieve this is to capture the pawn’s current target with a different piece and invite a recapture. Thus Black starts with NxN. White has to either forfeit the knight or recapture with his queen; and if he does recapture QxN, the pawn advance d6-d5 now has a very different significance: White is forced to move his queen (and he can’t move it anyplace where it can protect the bishop) and then suffer the loss of a piece with BxB.