It's a HTML/JS/CSS prototyping tool, so if you're building a web application for iPhone it makes it so much easier to get a basic prototype together and start hallway testing. Interface builder is _not_ the tool for doing this.
Additionally, just because you're not 'creating stuff like this in graphics' doesn't mean that you'll never create a proper interface for the application, it just means you can get a __prototype__ together quickly. Spending time creating a well designed interface takes away from what a prototype should be.