What's the Philosophy of glStencilFunc() parameters???

Does anyone know the philosophy behind the parameters GLint ref and GLuint mask in the call glStencilFunc.

I was wondering why the extra parameter: mask, and why is it bitwise “anded” to ref and stencil … ?

I have sought the answer to this question in so many places, but every one just quotes the redbook or the bluebook - neither of which appear to be in a hurry to justify the extra “baggage”

(… I know there must be very good reason for these parameter’s. That’s why I’m asking…)