"The strength of having a rook behind a passed pawn, is that it can also defend from the side" Beliavsky".
This quote was illustrated nicely in the Goldin ending. For those who do not know: It is best to have a rook behind a passed pawn. The second best spot for the rook is to the side of it. If you can not achieve the first two, a rook should be put in front of a passed pawn.