I understand someone likes them, but why are they even a thing?
I think regular music is good enough and artbook might be nice, but I'm personally befuddled by how many games have artbooks and soundtracks - do they even sell?

Also, I'm all for steam=gog update+ dlc parity, so this isn't against those dlcs, I'm just wondering why are they made in fist place even for steam users?

I don't know what percentage of games are bought with soundtrack DLC. What I find weak as hell though is the recent trend where "Remasters" of old games try to sell the same formerly free soundtrack as "paid DLC" vs the 10-20 year older (and 3-4x cheaper) original version of the game included for free. It's also been asked (but never answered) as to whether the artist / composer receives anywhere near as much for a soundtrack bought via GOG / Steam (after the publishers take their cut out of the 70-80% left after the store takes their 20-30% cut) vs buying an album direct from the artist on Bandcamp (artist generally keeps 85% of payment)...