This is an interesting review of the Mac Mini/Snow Leopard Server Combo offering.
I have a Mini at home in the kitchen and it does very well at web browsing and e-mail. It is also very reliable and has given me very few problems, so this may be the server of choice for the home and small office.
The review mentions that it has some limitations so for the small office you may want to think twice.
As far as "why the heck would I want this?" I don't know if I have a solid answer. If you want to do away with your Mobile Me account, it seems that iCal could run on this server. How about if you do development at home and would like a server to experiment with? FileMaker Server works best on its own server...
So if budget is an issue, maybe this is an answer. I wish I had the $1000 to try it out.