Quad Cab door still question

I had a buddy of mine help me with fixing some rust in the door stills of the rear doors on my quad cab. We cut out the rust and put replacement metal in its place.
What is the reason Toyota put the hole with the plastic cap in the rear door stills on the bottom? is this for moister drainage? Should I cut the holes again?

 

Might be an access for wiring or to reach a screw, doubt it would be a moisture drain or it wouldn't have the plastic cap on it...