From A Webkit-based Platform? - Mac Rumors:

However, Apple's inclusion of 'Save as Web Application' feature in Safari 4 could alter this reality. By bundling Webkit into a standalone executable, developers could theoretically release downloadable Webkit-based applications for use on Windows XP, Tiger and Leopard. To the end user, these would appear as standard applications, but the underlying technologies would be Webkit and Javascript.

The idea of packing a web page (or set of pages) into an application sounds wacky at first, but on second glance I believe there's a lot of sense there. For instance, having Gmail in its own browser that doesn't have to interact with my daily browser and web coding would be great. Perhaps you'd like to set up a browser for handling online banking with different security settings.

I hate it when browser content interacts, whether it's too many Flash adverts slowing down another window or a crash bringing down everything I'd been viewing. I also like closing out every window I've been using to Google down the answer to some coding question I had without blasting everything else. Often I'll have three windows of 6-7 tabs a piece with snippets of code that didn't quite work, and I don't really want to lose all that I've got open to clean them up. Now, I just open Researchilla and plug away.

So I've already been using different browsers for different tasks, and often have Safari, Firefox, and even sometimes Opera and Webmonkey open at once to eliminate any issues. This WebKit platform might make some sense in many cases.