My wife works for a local government agency in the US. *All* her correspondence is to be available for public review, if called upon. "I was using a private email address for all my correspondence, and I decide what is relevant to your request" does not work here. (I shudder to think at the security implications of all Ms. Clinton's correspondenceto foreign leaders going outside the Department of State firewall.)
Is my wife the head of the agency she works at? No. But the rules do not cease to apply once you're on the top. These are public records. They must be available, if not to the public at large, at least to *their duly elected representatives with requisite clearance.* Actions to make that impossible are highly suspect.
Do others do it? Yes, I'm sure. Are others this brazen? No, they're not. She's made an art of being caught doing something illegal and then managing it: first, by having her surrogates deny it, then appearing confused by the controversy, later indignant, ultimately blaming the usual suspects, and finally waiting for the mess to die down.
As for the reason Ms. Clinton did this: come now, let's not be naive. It's so she can selectively hide the contents of some of her emailers - the ones that she takes bribes from. Sorry - no other reason I can think of to set up your private email server to receive all your emails for years and keep it out of the government's reach.