It's all about link equity. I know that's not going to make any sense whatsoever to most of you, but trust me on that one.
If you want to learn more about the why's and wherefores behind it, send me an IM sometime and I will get into a little more detail for you.
Remember, I've been covering the
SEO scene for almost 8 years now... but I can't be spewing all my cool little tricks in a public forum.