Personally I think the main draw of Discogs is the marketplace, even with the level of scalping it has.
I think a good 50% of my local music is missing from Musicbrainz.
When the release is present, almost always it's missing different issues for the same release (e.g. the white label test pressing release, the final release, the restamp from 20 years later). Discogs almost always has all of them.
Discogs includes lot of useful information: is that specific release single sided? Was it pressed on acetate rather than normal vinyl? What had it etched on the runout?
I use mp3tag connected to discogs (auth tokens)
It's incredible - i feed it files, it spits out glorious tags, folder names (perfectly formatted using regex)
The only thing that imo is annoying is getting catalogue IDs and using discogs + mp3tag to bulk/intelligently provide catalogue number.
www.discogs.com
They have implemented a database with a pretty good representation of all the music releases (realizations, in other words), as submitted by volunteers (much like Wikipedia). How many of the questions posed here have already been resolved in a reasonable way by Discogs?
Then there was the late great Catraxx database, using much of the same relational structure as Discogs, but based on the MS Access engine. It can be used to write tags to audio files.