In our chess games often situations occur in which one side can trade two light pieces (six pawns) for a rook and a pawn (also six pawns). Nominally speaking, this should be equal, but in reality it is one of the sides who profits from the exchange.
The general rule states that the pieces are stronger, especially at the beginning and in the middle game, but this is not always the case.
If the rooks have open files and can create direct attack against the enemy king they might be the better pieces.
Have a look at the following example:
The black pieces on the queen's side never really came in the game...