There are a lot of reasons not to code in your free time. You spend most of your waking day in front of the screen already if you hold a computer science degree. Getting more screen time is the last thing your body and mind actually need. So why should you start a computer science side project?

Falling in love with coding again
Side projects can be a good balance to your day job because you have total creative freedom. No corporate code guidelines, no technical debt and legacy code and certainly nobody telling you which technologies to use. This can rejuvenate your love for computer science and in turn give you more energy for your day job.
Build new skills and experiences
You’re in full control of your side project. That means you can use it to try out new technologies and practice new skills. Those skills acquired can help you advance in your career. You can use your side project to learn new technologies which are in higher demand and open up new job opportunities.
Side projects are a great sandbox to generate escape velocity if you don’t like your current job.
Broaden your skills with no-code side projects
Side projects don’t have to be all about code. In fact you might gain more if you go for a low code side project like starting a blog. You can hone important skills like communication, SEO and marketing. Such skills are highly transferrable and desperately needed in about any job. Even if you’re not eyeing out a marketing job even as an engineer you could use your marketing and communication skills within a company to advertise and lobby for your work.
Create your personal advertising space
By building interesting side projects you’re buying yourself important advertising space. Many companies value an active Github profile because it says a lot about your dedication. Even if nothing ever comes out of your side project it still contributes to your career because it give future employers an important chance to get to know you and your skills.
Open source contributions are your street credits as an engineer. Your Github and StackOverflow profiles are free advertising space for you to use.
If application security interests you try some hacker bounties. You could then share in a blog post how you looked for exploits.
It might pay the bills some day
Just because many side projects never succeed doesn’t mean that it happens to all side projects. Some of your projects might turn into a nice side hustle and buy you a bit more freedom. Some others might even turn into a full-time business.
There’s a ton of ways how you can monetise your side projects besides online advertising or building a SaaS product. One such example is Github Sponsors which has turned out to be a viable income stream for many engineers.
A computer science side project can advance your career in many ways
You can use your side project to open up new career options by learning new skills. It can also refuel your internal fire as an engineer or help you discover a brand new passion. In the best case it can even turn into a viable business for you.
So if you feel like you should be starting a side project, you should probably listen to your inner calling and get building.