Why, exactly, should I get stuff mastered?
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.