Technically www.example.com is a subdomain of example.com, but hosts usually redirect both to the same content as standard as users generally think they are one and the same. But you could have different content on www subdomain if you wanted.
Chances are that this just hasn't been configured for some sites, so only www subdomain resolves. Potentially they could use the non www version as an internal-only admin or intranet or something?
my users often just type in "website.com" to get to my site. I have this redirected to "www.website.com" because I don't want search engines to treat the with-www and without-www address as two different pages; they're one and the same, so I might as well merge them with a redirect.