Beyond looking cool, it's often useful to keep track of system information. One idea that I know appeals to some people is that it makes it possible to put prompts on different machines in different colours. If you have several Xterms open on several different machines, or if you tend to forget what machine you're working on and delete the wrong files (or shut down the server instead of the workstation), you'll find this a great way to remember what machine you're on.
For myself, I like the utility of having information about my machine and work environment available all the time. And I like the challenge of trying to figure out how to put the maximum amount of information into the smallest possible space while maintaining readability.
Perhaps the most practical aspect of colourizing your prompt is the ability to quickly spot the prompt when you use scrollback.