It is perfectly valid and beneficial to have side projects.

First, it lets you experiment with new technologies and frameworks outside of your regular duties. (This can pay dividends to the employer)

Second, it can keep you from code burn out by giving you a creative release. (Also a benefit to the employer)

Third, it gives you freedom to experiment with smaller projects that might not be enough to support a person but create value for someone none-the-less.

Finally, one of those side projects might get traction and create a sustainable business.

Not everyone has the freedom to throw caution into the wind. And oftentimes, even if they do, it is not the best move.