My name is Ricardo Cecchi and I run Online Audio Works. A mastering facility set up to help independent artists release high quality music.

When you send your project out to a mastering house, one of the biggest benefits you are getting is a set of fresh ears to go through your mix with a fine toothed comb.
The mastering engineer will pick up on things you didn't (either because of your bias (it's your work) or because you have simply listened to it too many times) making sure the final product translates well on a variety of systems/platforms worldwide.

The mastering session is the last chance to get your project sounding its best before manufacturing or digital distribution.