If you have developed web based applications for… let’s call it “simple users” you know that sometimes not having anything besides the page showing can be an advantage.
Prism is basically a stripped down browser that runs webapps like they were desktop applications. At first it might seem useless, but sometimes people think applications that run on browsers are just websites and don’t have the same value as a desktop application.
For that kind of client, a solution like Prism is the answer.
The other good thing is that it runs on the gecko engine, this means you can focus your development on a decent browser and don’t have to bother hacking your css and javascript to work on IE.