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