Its to cause arguments among families and increase social pressures on those with weak principles.

This is the kind of thing that your annoying Aunt or Mom will bring up to cause a fight because she was already mad about your beliefs, and then keep the passive aggressiveness high in your house.

Which has the benefit of shaming you into silence about your support (making Trumpers seem isolated and low number), changing your vote entirely, or breaking up your family to keep their own supporters isolated and echo chambered.