Good question. I think it's because we almost always see these folks with their makup on, looking great. In comparison, we frequently see our friends, spouses and others without makeup. We wonder why they don't look better. So, we want to see HOW or what these celebs really look like under the pancake. Do they look similar to us, better or worse? I suppose it makes us feel a little more normal to know many of them aren't that much better looking than many people we know.
I know I love to see the skankiest portraits of my fav stars just so I can laugh and say "see you beeyotches, you ain't got dat stah qualitee alla time, beeyotches!" Then I go watch to make sure I don' laff so hard my mascara shee runnnnnn! eeek! vengeances, dudes, we get vengeance on dem richiees!