at the front and at the end for draining the water out from the door which comes down thru the slots between rubber and window. (This is what I know from my E32, but I guess it will be the same.)
Under the door there are these mud flaps/sills, just where they end there are these holes. This water also runs into these flaps and corrosion starts. Here is a pic from my car where I just started to repair the corrosion.