Side gigs aren't worth it. If you're at a decent company, my boss feeling slightly more enthusiastic at my potential could mean tens of thousands of dollars more in bonus/stock at the end of the year. I'd rather work harder on work.
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There are indeed other reasons for side gigs, but the parent question here does seem to be focused on the financial side of things.
I would agree that work-related progress is likely to be more profitable in the long run. IF (and it's a big IF) there is a route at work to more compensation then that is likely the best use of your time.
Of course not all jobs offer that path, and in some organizations it can take a bit of detective work to find out where things pay better, or where more value can be recognized.