westfw:
I've never understood why an old (free) PC wasn't a better choice, either.
Because it costs more than using something like an RPi.
Each of those PCs is going to have its own mix of peripherals, unless you're lucky enough to get a uniform batch from that call center that just got outsourced to India. So each is going to need its software to be individually configured. If you're using Windoze, there's a good chance that they'll be licensed for different versions. Which means they'll be developing their own particular problems that the IT person needs to know about and solve. Ask anyone adminning a mixed fleet of a couple dozen or more to tell you about it. But be sure you bring a designated driver, because you'll need at least 3 or 4 rounds of beers to get through the horror stories.
Plus they need constant updating and monitoring to keep your computer lab from turning into a platoon in a botnet.
And, since they're old, a significantly higher percentage of them will develop hardware problems that need to be diagnosed and fixed (or, at least triaged to decide whether they're going to be e-cycled).