Herb Sutter has very nicely explained it here.
Guideline #2: Prefer to make virtual functions private.
This lets the derived classes override the function to customize the
behavior as needed, without further exposing the virtual functions
directly by making them callable by derived classes (as would be
possible if the functions were just protected). The point is that
virtual functions exist to allow customization; unless they also need
to be invoked directly from within derived classes’ code, there’s no
need to ever make them anything but private