Reduces exposure to unintended changes.
A private class pattern uses information hiding to protect a class's data. In doing so it blocks a client from making it unstable by unintentionally or intentionally modifying its state.
Since it is built into most languages, it might not be considered a design pattern by some. However, for the languages that don't support it, it can be challenging to come up with a way to make it. It is useful to have a pre-designed pattern to use in these cases.