Once you have an excellent mix, you need it to convey the same emotional impact in every situation and across all kinds of listening devices, not just in the studio. And you want it to shine among the other commercial releases. This is achieved during the mastering stage, through a meticulous use of specific compressors, equalizers and other mastering gear.
Mastering not only sets the ideal loudness and tonal balance according to the music genre, but it also get your music checked, exported into the proper format for each output (CD, vinyl, digital distribution) and finalized before it is burnt or uploaded forever. These steps require a solid skill set and a reliable professional.