This article on the InfoWorld web site is simply fantastic. Whether you are at all familiar with SoftGrid, Thinstall, ThinApp, XenApp, SVS or whatever, or even if you haven't even heard the term "Application Virtualization", you should read this. The title "Leave your laptop behind" focuses on how to leverage AppVirt technologies to break the ties between application installations and computers, letting you roam about without having to tote a laptop along. Basically, putting your packaged apps on a USB drive and launching them in any Windows client. I agree that this is much more feasible with ThinApp than with SoftGrid (now called Microsoft Application Virtualization) simply because ThinApp (like it's predecessor "Thinstall") creates the app package entirely self-sufficient and therefore portable.
I won't go into AppVirt concepts or comparisons of products and technologies, there's plenty of that already out on the web. This article however puts it into a tangible, realistic scenario that should make it obvious what the technology (in general) is good for.