No idea if this is traditionally why www. is used, but one possible reason:
Say you have a server which runs web, SMTP and IMAP servers. Users access the web server via www.example.com, SMTP via smtp.example.com and IMAP via imap.example.com
Your server becomes heavily loaded, so you want to split the web-server to a new machine. To do this, you simply change the "www" subdomain to point to your new web-server's IP address.
For larger internal networks, this is an easy way to move servers around.. Just change the internal SMTP server DNS entry and all clients will automatically start using the new machine. No forwarding of ports to worry about
With internet facing servers, you would probably keep both servers on the same NAT'd network, and forward port 80 to a different machine, or use a load-balancer
There are benefits to retaining the "www." part of the URL for purely web-servers, particularly with regards to cookies, as Andrew Moore and this blog.SO post explain.. Plus if you redirect the non-www domain, users don't have to type it (and even if you don't, most browsers will try "www.example.com" if "example.com" doesn't work)