Why does my music need mastering?

The stage of mastering adds brightness, impact and consistency to your audio production before it is duplicated, replicated, or reproduced on the radio.
Mastering creates a better separation between all instruments, can give more punch to the bass drum, make a guitar sound with more weight, and achieve that a voice fits into the mix without being disguised by other instruments. Basically, although mastering cannot fix a bad mix, all the recordings released for commercial distribution have been mastered, simply because it is essential to be competitive.