I've actually been wondering about this -- are there any major ones that meaningfully provide other kinds of value for all of the computational resources they're using?

My understanding of blockchain as a concept is pretty surface-level, so maybe I'm just missing something obvious here. But at least at first glance, it kind of baffles me that so many of them are using seemingly arbitrary hash algorithms to calculate proof of work, when they could instead be tied to distributed projects -- folding@home, for instance -- and measuring actual work.