Why is black forced to move the king?


Several other moves are possible too.
That's my question right there. Why is Kf8 better than Qf8, for instance? This may be too theoretical for someone at my level, though. I mean, I'll be honest: I don't understand that opening AT ALL.

The computer keeps changing his mind about which move is best. The frontrunners are f5, Bh6, Bf8 and Kf8 (and not Qf8, for the reasons that others stated above). So Levy saying that Kf8 is "forced" is just weird.

Several other moves are possible too.
That's my question right there. Why is Kf8 better than Qf8, for instance? This may be too theoretical for someone at my level, though. I mean, I'll be honest: I don't understand that opening AT ALL.
After …Qf8 you’re likely to be forced to move the King anyway if white plays c5 with Nd6+ coming in. Another benefit of …Kf8 is Black could follow up with …Nf6, which wouldn’t be possible after …Qf8 (e5 pawn would be hanging)
Of course even after …Kf8, Black’s position is not exactly great.