mastering exists to prepare a consistent-sounding collection of material for distribution on particular formats. mastering is done differently for vinyl vs. digital (CD/download) releases, but it's about ensuring the entirety of the product, audio-wise, sounds like it belongs together. it's particularly important for collections of materials that were not recorded together in the same conditions, or were recorded with wildly different setups but intending to sound coherent together (if that makes any sense! sometimes it doesn't!). this is especially useful for compilation albums and artists whose sound includes a wide breadth of genres.
if everything is recorded professionally by people who know what they're doing – like SW – you can mix it so well that mastering isn't needed. it just so happens that most engineers, musicians, and producers aren't on that level (or, if it's your viewpoint, they aren't pompous enough to think they are!). Iron Maiden did this with one of their albums and while it isn't personally my favourite effort of theirs, it sounds consistent with their usual production, so they didn't miss much by bypassing mastering.
however, if your typical metal band records an album with Rick Rubin at the helm, somebody had better fucking get the thing mastered!