Or you could buy a reliable server with a quad-core processor, RAID 5 disks, and plenty of room for expansion for far less than the cost of a Mac Mini server.
But then you've got to buy the server software. Yes, you could set it up as a Linux server for 'free', but then you've got to pay somebody to set it all up and maintain it.
Small workgroups often purchase the Mini server because the server tools are just so easy to use. Apple have spent a lot of time on them. They even have three sets of tools depending on how technical you want to get.
That's what a lot of people appreciate and are willing to pay a little bit more for. It's what I'm willing to pay a bit more for, for sure. I'd rather spend my time doing something I'm good at and bill the customer for that. You pay somebody else to do the other things.