It was always necessary too put www as the start of a URL ( uniform resource locator or more commonly called a web address)
If it was put in a Google search box by mistake instead of in the top address bar it just gave confusing results and didn't find anything of use.
Things seem to have changed as of Jan 2014 but in the past, it was common to specify the difference via a prefix for consistency. So for example, Acme Industries might buy the domain-name ‘acme.org’, then set up one or more computers to host the different services they have. When you want to use one of the services, you enter the appropriate host name:
So why does it still work without ‘www’? Because most web-servers allow you to accept different URLs and then redirect them as necessary. For the convenience of users, most companies and organizations set up a rule to have the web-server handle connections to the hostname on port 80 (the “web port”), or redirect it to another system if the web-server is a different machine.