To make your website or application have its own name.

Basically all websites around the world have their own names and if you only have an ip address it is likely that people will remember it very difficult, therefore you need a domain or subdomain so that your website or application is easy to remember.

If you are building a new application or website you will need a domain to connect to your hosting.

because your hosting provider will ask for a domain name for setup the first time they set up your hosting.

Or if you use vestacp, webuzo , cyberpanel on your own VPS you must have a domain name first for the first time setup

This tool uses the Cloudflare API with this tool you can make any domain name for your website or application even cooler in the eyes of search engines or people who see it.