There are still targets to be met and demands still being made of you. Sure, if you have nothing to do, do something good with your time; that's not always the position people are in.

> Life is more than serving your boss in the office.

Sure, I don't think anyone's saying you exist to serve your boss. But if you have to work a job, why not find one you actually like doing? I think what the OP describes would be a complete drain of my energy, even if I could read a book occasionally; working on actual code during my workday gives me an opportunity to be paid to learn new things and is honestly actually energizing. Why work on a tiny Nix environment on a toy project when I could be setting up a real-world Nix development environment that's deployed across tens of developer laptops, y'know?

I'm not serving my boss. My boss has a whole set of metrics and expectations that are quite distinct from delivering working software to customers; I use my considerable autonomy as a white-collar worker to half-ass those behind her back and spend my energies on what is actually satisfying to me, which is to craft good systems and make things work well. And as the sibling comment mentions, this is a lot more interesting when there are real customers and stakes.