The essence of the answer depends on the context from which the question is asked. When a checker checks a domain for availability, then you do not need to use www at the beginning of the domain.
When a domain URL is issued after purchase, the use of www is up to users' personal choice. It is necessary to follow the initially selected type of link in order to follow it in the future. Opening, for example, at both http://www.vashsite.com and http://vashsite.com, the site is considered, in fact, as different web resources.
More recently, all addresses on the Internet began to be entered with www, these letters were also present in various marketing products, such as brochures and business cards, posters and other materials. Worldwide network has reached such a huge size that if you do not write at the beginning of the www, it will be a relief for a multi-million user audience.