The raid finder serves two purposes.
1) (in theory) it allows a group that is missing people to get someone from the raid finder that matches set criteria
2) it allows a full pre-made to enter the raid from anywhere (instead of at the raid entrance)

Had it been added in 3.2, maybe 1 would have been more common today, but since it was added in the middle of a raid cycle, it's not going to see much use.

Together with the future cross-server party finder, it's going to get better.