Mastering music is a process where polishing the music happens. This process can bring out the best parts of the music. The exact reason why mastering music is absolutely necessary to be done by every musician is because mixes which are done on the typical studio speakers often make the mixing engineer think that the mix is already good enough when it actually still can be enhanced to actually make the best out of it. That is why only mixing the music is not enough but also but it should proceed to the mastering process.
To do this process in creating professional music, it should be done in an actual mastering studio. The studio is where the true quality of the music will be revealed using better speakers, amps, acoustics, etc., revealing the flaws which still need more attention and the areas to be enhanced. That is why choosing a mastering studio to do this process needs to be done carefully that the musician is able to find the best one.