What is purpose of the slot at the base of my drill's handle?
I think Tyler Durden is correct. Have a look at the D25013K model; there are 4 similar holes on either side, with a separate lanyard attachment point. For what reason other than air circulation would you add such holes? Seeing that they also let contaminants in, there must be a good reason beyond good looks for placing them there (maybe the trigger switch units heat up?).