Strengthening the g6-pawn, and creating the threat of bringing the knight via h4 to f5. At the same time, the black bishop still hangs. Even if White was not totally certain that the move was winning, he needed to play it, as with such a domination of the light squares, he cannot possibly lose. From a practical viewpoint, the most Black can hope for is perpetual check, but in fact there is not one.
In the game, White grabbed the piece with 53.Qxc3, but after 53...Kxg6, the black king breathed easier, and the black pawns all round the board started to have their say.

Nothing is changed by 53...Bb2 54.Kh3.

There is no defence against Nf3-h4.

One can see the most beautiful and accurate mating nets in endgame studies. It seems that White clearly has insufficient resources to win this position, but by exploiting mating possibilities and the fact that he has the move, he manages to win:

1...Kf5 2.Qd3++–.

Otherwise, the queen is lost. But what now?

Drawing the king into the trap.

5...Kxh4 6.Bf6#; 5...Kf5 6.Qf6+ Ke4 7.Qd4+ Kf5 8.Qd3++–.