I have to disagree. I think everyone, no matter what their career path, should have a "side hustle," of some sort. It's in their best interest of survival. Trust me, the company they work for isn't in business for the best interest of their employees, but for the owners of the company. I do think it's a bad idea to have a side business as a vendor for the company you work for as an employee. That's definitely a conflict of interest.
But having a side hustle is important, so when the real job slows down or heaven forbid, goes away, at least you have a small source of income while you find your next job.
At my last job, the company I worked for occasionally hired employees to do vendor flat-rate work, but they only paid employees half the vendor rates. Eventually the company stopped allowing it because some employees would do the side projects on hourly paid time and claim the flat rate too.
I have several side jobs. I sell books on Amazon, I sell collectibles on ebay. I am a notary. And occasionally I freelance on my days off to cover vacations for a few landlords I know. I don't have any time for these side projects anymore, but it's nice to know they are there when I need them.