With a domain, you can not only point it to your site so visitors can access the site through the domain, but also make further use of it. Below are some examples:
Subdomains: With a domain (top-level domain), can setup many subdomains. For example, with example.com, can setup subdomains like www.example.com, blog.example.com, news.example.com, etc.
URL Forwarding: Redirect traffic from one domain to another domain, website or a certain webpage. It’s also called Domain Forwarding.
Email Forwarding: Automatically forward emails to other email addresses.
P.S. There is usually a Transfer Lock on the domain control panel. It can be used to protect your domain from unauthorized domain transfers.