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're really not that cheap anymore, though. If you just need "another computer" you can probably find something more capable in the used market.
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.