It's often used in libraries where you need to guarantee ABI compatibility. Fixing a bug or implementing a feature may require adding a new member into the class, which would change its size (thus break ABI compatibility). PIMPL is the typical solution here, since the inner/impl class is not part of the public ABI.

I also like to use it sometimes to "hide" private methods and their documentation into PIMPL, so the public header is kept clean.