The point is if Palestinians and Jews are both descendants of Canaanites, and especially if many Palestinians have some Jewish ancestry but are now Christians and Muslims, then one of the biggest arguments of Israelis falls flat.
This is one possible way Palestinians can argue to win over international audiences and counter the argument Israelis give, trying to use biblical scripture or genetics. Obviously some will never be convinced, but many liberal and scientifically minded people will see the Palestinians as the natives of the land.