In fact, domains existed long before the Web did. If you have it in your head that a domain and Website are somehow the same thing or very nearly the same thing, reboot your brain now and start over. Your Internet memory has been corrupted and needs wiping clean for a fresh start. Domains are used for giving names to computers on the Internet, so people and computers have a way of referring to computers that they can both understand. If computers are going to communicate with each other via a network, they need ways of referring to each other.
Computers are fond of numbers, so they use numbers to refer to each other.
The computer you got this page from is called 3326418085. You didn't have to know that because people use names to refer to computers. If your name is Fred you might name your computer fred. Nancy working at the next desk might name hers nancy. Then everyone would know whose computers fred and nancy are.