I understand the private profiles perfectly, they usually just don't want someone they don't know looking at their info, pictures, reading there comments and profile finding out stuff about them, pretty much people stalking them. Some people might not want their parents, siblings or some relative, maybe their teacher, boss or someone to find their profile and read some of the stuff they have on there.
Also celebrity type people might just have it private and for you to type in their name to stop those automatic friend adder people or people that just randomly add people to make their friend count go up. There are a lot of good reasons to have a private profile.
Now, back on topic....