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They don't produce any tangible products as far as I'm aware, just intangible stuff like artwork, games, music etc. They just give your colonists something to do once you've built your colony to the point where you have tons of surplus resources, and people without any other useful jobs.
 
They contribute to a milestone and increase the comfort of their workers.

Otherwise, they literally exist to consume resources and create unproductive employment.
 
technically they also boost the morale and comfort of anyone working in them, which is only really useful for if you want to play short on services or increase birth rates... Or if you have too many machine parts/electronics and need to get rid of them so your Mohole Mine can produce more rare metals/metals.