Black cannot be allowed to penetrate on the K side, since white is moving his K away from his K side pawns.

? This creates the weaknesses on the K side that white wants. This pawn should go to h6 instead.

With black's king on the Q-side, white will continue with his plan and pick up the isolated f and h pawns. But the b4 break must be prevented, so white's K must still go to c3.

After 48.Rh6 b4 gets messy, but white should still win.

Now if instead 48 ... b4, then 49.axb4 a3 50. Rxf7 followed by Ra7 is just in time to stop the a pawn, and white now has two passed pawns of his own. Compare this with the position after 48.Rh6 b4 49.axb4 a3. The win is not so obvious in that variation (probably 49.Rxh5 instead of axb4, but it's still messy). This is why white played Rf6 instead of Rh6 the previous move.

The h pawn won't get away, so the K continues moving towards c3. If 50.Rxh5 black can find some counter-play with 50 ... Kc4

or 51.Kc3 followed by Rh5 is equivalent ...

With the white K on c3, black now has no counter-play, and is lost after the eventual Rxa3.