I had a long response written out but it didn't go through, now I am annoyed and will be short and to the point.
This is useful because you can chase a ghost replay and adjust your driving to try and beat it. It's a visual representation of a time based on a saved replay. You can't collide or affect the ghost in any way, it is just a replay on its own track, your opponent is still the clock just with a fancy marker to battle.
Check out Forza 4 or one of the others in the series, their Hot Lap mode has ghosts which are really useful for tweaking/adjusting your technique
I think this would be great, it should 100% be implemented, just a replay hotlap mode