[tennisgal_001]

Yes, I also noticed that he makes challenges just to save some extra energy and he knows from the beginning that the call was right. Good tactics it is for sure, maybe this is the reason....

Another could be, and it's not just Federer who does it I think, but a lot of players challenge with the preknowledge that their shot is out yet they use the Hawkeye system to determine by how much their shot missed, to help their timing, placement, or simply out of sheer curiosity.