We know that. The GP is pointing out a secondary purpose that is often overlooked. Automatic door closers are subject to failure, and when they're inoperative then having a handle is a good thing. Commercial building owners also like handles because they can be chained shut easily if the tenant stops paying rent, even if the tenant has changed the locks.

This is not to dispute the (great) value of the book, but to point out that there's often more than one use case.