And sometimes you want to actually write code every once in a while instead of scribing another three-hour quarterly planning meeting during mandatory training week. Being able to work on your own projects at your own pace and make something out of it is incredibly enjoyable, even if it takes some of your productive hours away from work.
I guess what I'm saying is: hobbies are good, too.
It depends on the nature of the side project (vs. side hustle), its relationship to your day job, and your employer's rules (and attitude). In my case I've done a number of tech-related books. My employer knew about (and encouraged) them. And we've had a tacit understanding that I wouldn't be doing these purely on my own time and I'd continue to work on my "day job." It was never a problem and worked out for both of us.
I certainly know people who do similar things around open source projects.
I guess what I'm saying is: hobbies are good, too.