Sure Photoshop has always been hailed as the cat's meow of graphic design programs for the web. You might even go as far as calling it the inddustry standard. It's really good for some things, but the efficiency and workflow of Fireworks is considered by some to be much greater. Fireworks is specifically designed for creating web graphics. It's inherent workflow is much more user-friendly than Photoshop. It's better for creating web layouts and doing batch image optimization.
The pages feature in fireworks allows you to create multiple pages of a website. Sure, you can accomplish this with layers and folders in photoshop, but it's much more diificult to organize. The pages feature in fireworks is seperate from the layers funcitonality. It even has a master page functionality that allows you to have one aspect of your design, say, a logo and a main navigation bar, appear on all of your "pages" so you only have modify one file and it changes that aspect of the design on all of the other pages.
The assets functionality in Fireworks is also really useful. Let's say that you have nicely designed form elements that you like to use again and again. In Fireworks, you can save these elements, ie assets, and drag and drop them into your interface. Super useful for making interface designs on the fly.
Although I don't use this functionality personally, Fireworks also has the ability to generate HTML/CSS pages without actually requiring coding.