RE: face cache referred to but not explained

[Drew Adams]

         "This function clears the face cache for all frames.  If UNLOAD-P
          is non-`nil', that means to unload all unused fonts as well."

        The doc simply repeats the function name, without adding
        anything useful.

    It would be useful to add a little about why someone might
    want to clear the face cache.  I can't write it, because
    I don't know the answer.

    If nobody knows the answer, that is unfortunate but no disaster.

Can we at least get a description of what the face cache _is_? (What it
does? What it is for?)

Thx.