I know how www.example.com can be different from example.com. But is there really a need? Most of the time we end up setting both, www.example.com and example.com to the same IP. Is it still used only to make it more readable!?
I guess it's because people are so used to typing it in. You could do what twitter does and redirect www.example.com -> example.com, eventually it trains people not to put it in.
The only technical reason I can think of is that you cannot have CNAMEs for root domains, thus if you're using a CDN to host your website then you usually have to have it going from www or another subdomain.