Funny, I just came to the forum to ask the exact same question.
I'd like to initialize a PropertyGrid based on an object I already have in memory on the client side. I have to call the Dialog with this grid on it several times and I don't want to make the user wait every time to fetch the same data from the server.
I was also looking at the ComboTree this morning, and that seems like something I'd much rather construct on the client side than in my RDBMS.