The seventh rank is where the opponent's pawns begin the game, so a rook that reaches it usually gobbles material while restricting the enemy king to the back rank. Reaching it normally requires first controlling an open file, which is why open files and the seventh rank are studied together.
Doubling rooks on the seventh is one of the most crushing formations in chess: the rooks defend each other, harvest pawns, and can generate mating threats against a king trapped on the eighth rank. Even a single well-placed rook on the seventh frequently provides enough activity to draw or win an otherwise difficult position.