As he said, reduction of static friction.
A great many skis come with graphite in the PTex for the same reason. That's why the bases are black. There may be some theorizing about heat conduction and radiation involved in the black bases too ... I'm not sure.
As for graphite in wax, the intention is apparently the same. See, e.g.:http://www.dominatorwax.com/techedu.html
Dominator, and others, also market waxes with graphite which are supposed to "renew" the graphite in your bases. Some brands use molybdenum instead, for some combination of technical and marketing reasons.
As for whether the bases really need to be "renewed," that seems to be an open question. The "Backshop" article in the current Ski Racing
says more-or-less the opposite: that bases need to be cleaned after using graphite waxes, because graphite waxes tend to clog the base's pores. This sort of diametric opposition is surprisingly common when you start reading about ski wax, and ski care generally.