It's really just for the purpose of looking cool. Modern CPUs include hardware random number generators that work just fine.
But it is also a source of entropy that is hard to manipulate (if we ignore the cameras, image generation etc). And it is a good source, initiator for discussions about entropy, random numbers and why we need them to secure things. This thread on HN shows that it is so. And for 'normal' people probably even more.
I'm sure they have a lot of non-geeky visitors that look at the installation, read the sign and start asking questions. And if it was me, starting to jump in front of the installation, waving my arms - to provide my bit of entropy to the pool.