A domain name, in its simplest form is your Internet Website address.
Just as your house or apartment has its own unique identifying address for the post office, so does each and every Website in existence.
Your custom domain name is your business address on the internet.
As with any business address, this is how and where clients and potential customers can find you. This is also used for receiving and sending your mail.
While it is entirely possible to have a website while only using a sub-domain (e.g. "agent-jane-realtor.myrealpage.com"), this can often be viewed as unprofessional, as compared to having a custom domain name (e.g., "www. agentjane. com").
As per Wikipedia, "domain names are used to establish a unique identity. Organizations can choose a domain name that corresponds to their name, helping Internet users to reach them easily."