I love the freedom in a side project to write a thing, then rewrite it, then decide on a new requirement and rewrite it again. No deadlines, no stress, just incrementally experimenting until I'm happy.
At work they rely on me to deliver in a reasonable time, and move on to the next task. Once something is working, it generally isn't changed too much, even to improve it (obviously if it's really important to improve it we make time for that, but that doesn't happen so often)
I think it's nice to be able to write something well, polished and sturdy. Something better than at work. higher ideals.
Or, to write something that is house of cards nonsense that would never fly at work but does something fun. You don't have to explain. Sometimes not having to explain is the BEST.
Oh absolutely, the real win is being able to play with a concept with no risk