But why does a domain need to host a webpage? I get that we mostly want snazzy domains for webapps these days, but if I grab a domain, point it towards a server that only serves SSH externally, what's wrong with that (Actually, I am considering doing just that for a domain I have been eying).

Defining 'substantial' is difficult, especially if we start pulling lawyers out to sue for the lost of virtual 'real estate.'