Mastering creates a better separation between all instruments. It will give the kick drum more punch, make the guitars sound fatter, and the vocals will flow with the mix without being masked.

If you compare your music with your favorite CD you will probably find that your pre-Mastered mix sounds sonically inferior. Your mixes may not be loud enough, or the vocals may be to upfront, or buried in the mix. All Recordings need Mastering to fix problems within the mix. Even if the mix is "perfect", you will still need a Mastering Engineer to adjust the volume levels to be competitive and consistent.