A Raspberry runs an OS, there are quite a few ported already. It has ethernet, USB host, HDMI out, micro USB, an SD card slot and GPIO’s. It is a complete computer, connect a keyboard mouse and monitor, use an SD card with raspbian or another OS and you boot a computer, not so powerful as the intel and AMD desktops but still. It is used as media center, streaming video, or to surf the web, as VPN server, NAS or to edit documents, teach kids about coding, home automation, software development, webserver, …. Just a few examples, but the pi is used in a huge number of possible use cases. Some have combined a number of Pi’s to make clusters and make a kind of Pi supercomputer.
How can Raspberry Pi be useful?