There are way better uses for old laptops, such as using it as a homelab, media server, etc.
I have a modern laptop and desktop, so I simply do work on the laptop and play on the desktop. Not everyone has that option.
It's also possible to have an old unused laptop without having a newer machine. Not very likely in this case since the author seems to be very much interested in computers, but there are certainly people out there who have replaced their computers with mobile devices and may not even touch a traditional computer (outside of work).
- it's more fun to have a device dedicated to one thing (or turned into a toy)
- just the act of making one is fun
Of course, you could also just plug in that new laptop to either of those things, but sometimes having a dedicated device for something is just more convenient, especially if you've already got the hardware laying around going to waste.
I prefer a secure OS: My main machine runs Qubes OS and protects me from all practical attack vectors. As a downside, it doesn't support any games.
I frequently play on my work M1.