So, the DNS converts host names into numbers. What use is that?
Well, humans tend to like names, they're easy to remember. Computers, on the other hand, like numbers, and since the Internet runs on computers it does things their way. The DNS is provided purely to make things easier for us to understand (it's a bit like The Matrix - you can watch the numbers go past if you really want to, but its much easier to look at the translated version).
Right now, remembering an IP address isn't too difficult, after all it's just four small numbers, but that's going to change. Very slowly people are starting to use the new, longer IPv6 addresses (up to eight, rather larger numbers).