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(I get it's math-related, but what is it useful for?)
The other main use case now is educational - I definitely would have liked a site like this when I was taking my intro to topology class and grappling with examples to get my head 'round things, and I know of a seminar that ran using the db as a hook to get students being self-directed, exploring, and contributing more than undergrad mathematics usually allows.
Admittedly, that may not seem "useful" unless you're a topologist.