At our software company we used to use PlayStation 3 as a development server running YellowDog Linux. Just testing our RPM .spec files was a gargantuan eye opener as to how many things we got wrong when we'd try rpmbuild --rebuild on YellowDog, and just because we'd write the .spec file on a PC-bucket server running CentOS. Monoculture is an extremely toxic, blinding thing, and it breeds ignorance.
Not everyone has spare, fully functional laptops lying around “free”.
By the time my first kid was born, I’d bought a new computer...well, a whole more than two times. And he’s five, and I’ve done more than two since. But the ones that aren't repurposed and in use also aren't reliable (that's why they aren't repurposed), and while you don't need much performance to learn (a plus for the Pi because it's not a master of performance), having something trustworthy is valuable. (Sure, in some cases the reliability issue is fixable with replace parts costing more than a Pi, but then...)
I haven't used one, I'm considering getting one for my oldest kid when he turns 7 next year, so it's really interesting to hear people's experiences in this thread.
[1]: https://www.raspberrypi.com/products/raspberry-pi-400/