Well if you do woodworking you finish a project by painting or staining…If you write a book or article you (do well to) have it proofread and edited before publishing it. Videos and photographs are color corrected. Cakes get icing, Ladies get lipstick, shoes get polish.
Mastering is typically (best) done by an objective audio engineer not otherwise connected to your project, This engineer makes sonic adjustments to meet technical standards, match genre trends and preserve and enhance the artist’s vision. Mastering is why rock sounds loud, dance music has heart skipping bass, and mostly why MY music which is self-produced in my humble basement studio sounds close to music produced by award winning professionals with half a million dollar budgets.
Mastering is worth paying for. Good mastering is worth paying more for. There is nothing more deflating then having a song you poured your life into sounding awful when a listener hears it – especially when played alongside other music. Sometimes you only get one shot at reaching someone with the message of a song. The fewer obstacles, the better.