how are rooks on files used

amazingly...

Rooks are used on files to take control of the file, effectively banning the opponent from 1/8 of the chessboard, and to get deep into the opponent's position; the back rank for a mate, or the seventh rank to engage in brutal, no lube chess intercourse with the opponent's pawns. 

As with any piece (knights, bishops, etc) the rough hierarchy is:

Best: pressuring a weak pawn, piece, or aiding in mate
example: a rook on the 7th rank

2nd best: influencing many empty squares, with hopes of achieving a "best" rating in the future.
example: rook on an open file

3rd best: defending an important pawn or empty square (will likely need to be improved later in the game).
example: rook behind friendly pawn

Of course, sometimes looks can be deceiving. For example

White is very happy with his rook on a1. White to move would play a4 to open the file for his rook where in an ideal scenario it could infiltrate to a6 or a7 to pressure weak pawns.

Similarly black is happy with his rook on c8. Black to move would play c5 to open the file for his rook where in an ideal scenario it could infiltrate to c2 to pressure weak pawns.

Another use for open files and Rooks is the "Rook lift" or "restricted advance in a file". The Rook uses the file to advance to a rank that is not obstructed by Pawns, and enters the game this way.

Example: