The obvious usage of privacy is so you don't expose an API you don't want to maintain. It's also occasionally used to maintain some invariants; for instance, yo

The obvious usage of privacy is so you don't expose an API you don't want to maintain. It's also occasionally used to maintain some invariants; for instance, you can have a public method that wraps around some protected interface that does input checking/clean up. Having the compiler enforcing privacy, in this case, guarantees that whatever actual method you dynamically dispatches to has the "right" input (accidentally calling the interface directly won't compile).