I for one really enjoy Hackdays. It's a chance to do something not in your normal line of work and experiment in a different area, or with different people, sometimes even different places (but often remote pairing).

A couple of hackday projects were proof-of-concepts that eventually made it to prod, in a very organic way. Other teams would learn what was done and adapt it for their needs. They do sort of play out like R&D playtime. I was so lucky to have started my career at companies that actually did software Research as well as Development.