The ability to create and share with a client a working prototype of their website is wonderful - all to often I find that clients are not able to visualize how a website will work unless they can get in there and use it.

Great for high fidelity wireframes and interation flows for the product.

For wireframing and prototyping, Axure offers the power to set variables that persist from screen to screen. It's really handy when building an app that requires some user input and a system that responds to input data.