I've been writing code professionally for quite a while now. I'm not trying to land my first coding job. The vast majority of programmers working on side projects are already established in the industry.
I've learned a few novel things from my unfinished side projects, but I've never really used them in my day job.
> The vast majority of programmers working on side projects are already established in the industry.
I strongly disagree. There are teenagers writing custom skin mods in Minecraft by Googling and copy/pasting. There are non-tech professionals making hello world android apps but are too novice or uninterested in getting a professional developer position. There are photographers who write plugins in Photoshop to create copies of their photos at various resolutions. You won't find any of them making github contributions but they're there.
And a lot of the skills you learn from side projects are internalized without being able to explicitly state it, like getting a feel for conventions in the framework or language you're using, or how much abstraction to use in your solution.
I've learned a few novel things from my unfinished side projects, but I've never really used them in my day job.