You can earn a higher hourly rate in your free time than you can earn in your day job. This is assuming that you have some sort of skilled gig you can do quickly. For example, I do IT side jobs after work and on weekends. The hourly rate I can charge for these services is way more than I would earn as a technician for somebody else.
Similarly, if you are skilled in some kind of trade or technical work, it is very likely you can do side gigs at a higher hourly rate than your regular job pays you. Typically, you would pay somebody a higher rate for short work than for long-term work.
There is some ethical consideration if you are undercutting your employer. I recommend against that. The best way to do side gigs is if your day job is unrelated to your side gig. If your side gig is in the same field, then it should be noncompetitive with your day job. For example, you may work as an in-house network technician for a company. Since your employer is not selling your services to the public, there is no problem with you offering those services elsewhere.
One of the toughest things about being self-employed, exclusively, is that income can be erratic. For this reason, self-employment is better as an add-on to your day job rather than a replacement. Your day job can pay your bills. Your side job can be your wealth creator.
When you are self-employed, it is often tempting to take any job that comes your way just to make ends meet. This is problematic because you may not charge enough just to get the work. On the other hand, if you have steady income from a day job, you can turn down side work that does not pay well. Having a steady job gives you the luxury to cherry pick the side gigs that really pay off. This means you work less, earn more, and have the ability to focus on quality rather than rushing the job because you didn’t charge enough.
To recap, having a day job that pays decent can help you cash in on higher-paying jobs after hours. Your day job gives you freedom to pick the best side jobs because you are not so hard pressed to lower your prices. If you are going to give up your evening or weekend, it’s going to be worthwhile.