Your domain name is like your address and your web hosting is like the house that you live in. If you want to launch a website then you’ll need both a domain name, which people can use to find you, and a web host, which stores the actual website.
Domain names and web hosting have a sort of symbiotic relationship, like software and hardware or yin and yang. While it’s technically possible to have one without the other, they work best when they’re working together, which is why many web hosts offer domain name registration and why many registrars offer hosting.
You can separate the two, of course, and register a domain name through one company and buy hosting from another. But it’s usually more time-consuming – not to mention more expensive – so why bother?