Black has two passed pawns on c5 and h4. The h4-pawn is ready to move forward, it is supported by the rook on h8. White has one passed pawn on b7, but it is blocked by the black king and it is hanging. In rook endgames, the activity of the rook is very important. So after Kf7-f6, there would follow 1.Kf6 h3 2.Kxf5 h2, where white has to do Rh1 and the white rook will become too passive. What else can white find here?