Say I'm setting up a single web page offering a service (will be a subdir of another domain, e.g. http://example.com/myservice/
). Now I've got the choice of either setting the page up under my personal domain name which has a very low page rank, or setting the page up on a completely unrelated domain name which has a much better ranking.
I would prefer the former, having it a part of my personal web site, but if putting the page on a higher ranked domain would automatically give the page a good page rank then I'd do that.