The Raspberry Pi IS a computer, but is a lot cheaper than a PC or Mac. It doesn't come with a DAC, so Stefan added one. Note that it may be of better quality than the DACs that come with PCs. Also, Stefan may have wanted to use a dedicated device for his music, rather than tethering his PC to his stereo every time he wants to listen to music, especially if his amp is in an awkward place. I'm getting around those issues by creating an AirPort network for my house. Note, though, that each set of speakers needs an AirPort receiver; that's why I was enquiring above whether a Pi can be adapted to work as an AirPort receiver. It looks like it can, with a little work.
I've just ordered an Pi for my son and I to assemble over his summer break. I think it will be good for him to follow the directions, see what doesn't work, get frustrated, consult the documentation and web sites, and get it right. After that, he may want to tailor it to fit any particular projects he may envision.