For the most part, the Pi is just an ultra cheap, ultra small, ultra low-powered system. It runs a specialized version of Debian which is the Linux distro Ubuntu is based off of so there's a lot of similarities. In general, unless you're wanting to run ARM assembly code, your full-sized PC will be better. The Pi was designed with the intent of providing a cheap computer for teens to learn to code and use Linux on. There's not much "special" about it other than the price.
Apart from the GPIO ports and the nifty ability to use an old TV as a display, the big thing the Pi has going for it is that it is totally quiet and consumes very little energy. A full-sized PC will normally have a noisy fan, put off a noticeable amount of heat and consumes (when sleep is disabled for server use) a noticeable amount of power. If you have a small home/office that needs a print server and/or a low-traffic file server, a full fledged PC is kinda overkill. If this sever won't ever have much traffic, a Pi can be run 24x777x365 with a negligible impact on your electric bill.