> At a deeper level, I always wonder, what [besides mining bitCoin] would other people hack up with a monstrous amount of computing power on the order of a data center in a container?

I wouldn't call it "hacked up", but large-area surveillance radar systems like AEGIS and JSTARS use very high density systems like this, because the size, power, and cooling available for the signal and display processing are severely limited. Same can be said for systems like the mobile Doppler radars the National Weather Service and various universities use for studying severe weather.