After listening to your tracks countless times in a row, it's hard to make precise, objective decisions about the minute details. A mastering engineer has both the experience and equipment to make the various tweaks and adjustments (balance, stereo image, EQ, etc.) that add up to make a big difference. There's a whole lot more that can be done in today's modest home recording studio than used to be possible, but there are some things that really are best left to the thousands-of-dollars setups of mastering engineers. For a review of what mastering is, see our post - Mastering a Song – What Does It Mean?
Read more about audio mastering and its benefits here: http://www.audio-issues.com/mastering/audio-mastering-important/