They provide additional information. Not only do you have the base genres like you do in the other services, but they also have reviews, styles, and moods. They are also far more complete and accurate than the free services.
I really like musicbrainz, but its information is often limited or incomplete. Discogs I only use as a secondary filler if the main information is already verified off of musicbrainz as i find it often messy and inaccurate.
I definitely would prefer if everything was soley available on musicbrainz, unfortunately a lot of stuff I find is not available. If I have the original CD then I will typically submit it. For stuff that I did not rip myself, I typically do not as I don’t want to submit anything that I am not 100% sure is exact.