White's c3-knight is threatening to capture on e4 hitting the queen on f6, and taking advantage of this diversion to retreat safely to c3. Were the queen not on f6, Black could turn the tables after Nxe4 with ...d5! picking up a knight. So the queen must move again. In the old days of the Leonhardt Variation, 5...Qg6 was the move, defending e4 and pressuring g2. The modern preference is 5...Qf7, renewing the threat of ...d5.