Pis are a relatively quick and cheap (including power) way to get "another computer" that isn't a VM or otherwise dependent.
VMs can add a lot of complexity that you don't really need or want to manage.
And (perhaps unadmitted) lots of people bought Pis and then searched for use cases for them.
They never were terribly cheap, but there was a time when people who hadn't seen a single-board computer (that small) and didn't really keep abreast of the used market gobbled them up.
One advantage over the used market is that you can easily keep getting the exact same one over and over again.
>They never were terribly cheap
Not really, no. Not if all you need is "a computer". They're more interesting if you want a cheap ARM computer, or a tiny computer, or a server that sips power. Although for that last one they're also kind of dubious. If you can somehow get away with running what you need off a phone, that's way better value.
They're really not that cheap anymore, though. If you just need "another computer" you can probably find something more capable in the used market.