Mastering is such a misunderstood thing. Maybe one of the pros will weigh in here with more authoritative info. But to me, there are a couple of aspects. I agree with Eric that one of the biggest benefits is just a second set of ears. If you go to a good experienced engineer, you're getting someone who has critically listened to a lot of tunes, and knows what sounds good out in the wild, and they can make tweaks - hopefully relatively minor, to make a track sound its best on all the different platforms it's (hopefully) going to be played on. They probably have better gear and acoustics than you do, as well. but the critical part is the "second opinion", it's like a QA step for any product release.
The other aspect has to do more with if you're releasing a CD, the mastering step includes sequencing, making sure the songs have the right spacing, the right relative levels so the CD flows. There is meta data to be embedded, ISRCs to be assigned, a DDP file to be created, so on and so forth.
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