My guess would be that it's a side-effect of evolution, in that play is helpful for the young and is driven by it being fun. Doesn't seem like adults playing is necessarily beneficial to survival, I'm curious what the evidence to the contrary would be.
One non-inconclusive piece is the massive number of productive, but opaquely so, behaviors that animals have. In this case, my guess would be that their "play" is closely intertwined with their intelligence, and that "play" is actually an evolutionary mechanic to explore potentially unoccupied niches in the environment. I.e. if "crow snowboarding" was somehow an exploitable niche, this crow might have just found it, and over generations, crows that are specialized to do just that would evolve.
This is a bad example, because I don't see a niche to exploit. As a more appropriate example, New Caledonian Crows use shaped pieces of twigs to dig bugs out of trees because their beaks are too short to reach. I could absolutely see that having evolved out of a young crow messing around with a twig. Older crows see what's happening and decide to try it themselves, and you end up with crows specialized to use twigs like that.
Imo, it's hard to find truly purposeless activities in the living kingdom. Millenia of natural selection have favored creatures that do purposeful things. The squirrels that liked to do nothing or romp around in the trees for no reason didn't spend their time stashing away nuts for winter, and they died. Humans largely being the exception, since our use of technology has created such a large ecological niche that we're not really at threat of being outcompeted or starving during the winter.
This is just an assertion, and it's actively contradicted by huge piles of evidence. Why do you believe it?