dehum for the basement, definitely. Removing the moisture will help slow the growth of whatever mold & mildew is down there.

If there is no obvious switch for the light then my guess is that some previous owner removed the switch perhaps because it did not work. Is there a blank wall panel somewhere near the front of the house? Perhaps a double or triple switch plate with one switch missing? You'd probably have to trace the line going from the lightpost back into the house to figure out where it goes.