That's a terrible idea. People would just create fake purposes for the websites.. and then who would be responsible to decide if a purpose is legitimate or not?
Wouldn't it just be a return to the days when domains were free? Back in the mid-'90s, when people I knew registered vanity domains, they tended to come up with some ridiculous expansion of the name to make it look official. Nobody ever checked that "foo" really meant "fraternal otter organization" (or whatever), so those would become the top line in whois.