In the immortal draw, why is the Queen sac 9...Qxa4+ considered the best move? After White's first nine moves in the Immortal Draw:

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1.e4 e5 2.Nc3 Bc5 3.Na4 Bxf2+ 4.Kxf2 Qh4+ 5.Ke3 Qf4+ 6.Kd3 d5 7.Kc3 Qxe4 8.Kb3 Na6 9.a3


Why is 9...Qxa4+ the best move here?  Doesn't Black have an advantage with White's King on the third rank?  What hidden (from me, anyway) resource does White have that causes the Queen sacrifice to be necessary?
Where has it been mentioned that 9... Qxa4+ is the best move? but considering the alternatives it is the move that has made the game immortal, suppose black would have just followed up by playing positional chess only and managed to win, the game would have become just a case of positional example and hardly immortal. The rest of the game was not easy for white, number of times a playing a logical move would have lost white the game.