Disclaimer: professional in this case means making a living as a web developer.
This is a question I asked myself and discussed with other developers over my career multiple times. It seems like most web developers have at least one private side project ongoing or they are planning to.
For me personally I also felt some need in the past to have a side project because all the cool kids I follow on Twitter have some and they have fun with it and are successful, at least that is what it looked like for me. And I wanted to have fun and be successful too.
If you were on the job hunt in the last years as a web developer or talked to some devs that were you might asked yourself the same. It often even feels like if you do not have an open source (e.g. GitHub) account and add that to your CV nobody will even look at it.
So that specifically developers that are new to the profession get the feeling there is a need for private side projects to become and stay a web developer, is no miracle to me.
From my experience, after more than 10 years of software development, private side projects are not a good indicator if someone is a professional web developer. Over my career I had years with and without side projects and I wasn't the only one, even the cool kids take breaks from their side projects.
But I will say that private side projects can be quite helpful if you want to improve your coding skills or want to get into some new technology because you can do everything on your own terms.
They are no requirement though.
And please do not forget there are web developers out there which do not have any private time to spare for side projects and this does not make them any less a professional web developer.
What are your thoughts and experiences with this?