in the past, the WWW was needed to define the internet protocol for the address. Technology has transcended this and now a good number of addresses with modern servers can do without the WWW . however some websites still operate with the NEED for the WWW, so that is why.
I know that some websites will ONLY work if there's a www in front of it. Other sites will work with both www and nothing and then there's some sites where www will not work.