In fact, the domain WAS an IP address. There are 2 types of IP addresses: one for local networks (you've probably seen this before: it's usually something like 192.168.0.X). Then, there are global IP addresses, which the Internet uses. They look much more random. You can find yours using several services available online. Now, back to WikiLeaks. That was the address of the server on which WikiLeaks is kept. And you can access any website this way: for example, Google is 188.8.131.52. You can get the IP of any site by going into command prompt on Windows and typing 'ping www.example.com', or the same command in OSX.
It's much faster because normally, you put in the domain. The computer then initiates a search across the web for the given domain name. When it gets an exact match, it pings the site to retrieve its IP address. Then, the browser uses the IP to access the site. On the other hand, if you just put in the IP address, it brings you directly to the site.