Firefox Private Browsing Windows...Gross?

Mozilla's Firefox has jumped on the bandwagon along with Google's Chrome, by updating their browsers with "Private browsing windows." Check out Gizmodo.

I am having trouble really seeing the purpose of private browsing windows...Well other then the obvious, spousal deception. I mean really what is the point?

So for those that done know what a private browsing window is, I'll explain:
1.) User opens Firefox.
2.) User gets an inkling to browse in "private". ooooooo.
3.) After going to several website of ill repute, the user needs to make a quick exit from browser.
4.) User closes private window, and the past (cache and history) is in the past.
5.) User can go on living life without feeling dirty?

Ok, so maybe its not porn...maybe its facebook. Either way it seems pretty fishy.

I don't want to knock Firefox too much though. It is the best browser I have ever used and there are a great deal of other enhancements (pardon the pun) that make internet browsing better.

...as always, tell me what you think.