I listen to pretty much all my music on my headphones. That is a whole lot of mastered, finished, polished, professional music. As a result, I know my cans inside out. I know how much bass they should respond with, how much treble there should and generally I know what music should sound like through them.
- Perfect listening environment
People tend to complain that headphones dont give an accurate responce, certain frequencies are hyped, stereo placement isn't good etc etc. But the one advantage they have for most home studio owners is that they dont suffer from reflection issues or peaks and nulls in an inconsistent way. If your room is quiet, headphones sound the same wherever you are in the room. As for the disadvantages, i've usually sorted the vast majority of those issues already by the time I come to do my mastering. Im really just making a few final eq changes at this point and doing a little bit of mastering compression and limiting.
- The microscope effect
Because headphones typically put the sound right at your ear, you can generally hear tiny details in the music. This is generally what you are looking for when it comes to mastering, the details, the small things that polish a track. As such headphones really are perfect for this task.