The main reason is portability and translatability.Â  A mastering engineerâ€s job is, mostly, to ensure that your final product sounds good everywhere.Â  Along the way that means punching a few things up, evening out EQ, adjusting loudness, managing dynamic range, etc.Â  It also means prepping the music for the final delivery medium â€“ CD, Tape, Vinyl, etc.Â  Itâ€s the final polish, and the final prep involved in getting your music where it needs to go.