I actually made the holes originally to secure a piece of angle aluminum across the frame, underneath the trailer seat fabric, as a way to repair the child carrying functionality since the original strap supporting the bottom of the fabric seat had broken previously. The seat sags way down without it, but the frame cannot be folded flat with the angle aluminum permanently secured. So I had to have something removable.

Then I re-purposed the holes to provide a way of securing the plywood to the frame. I have to be able to remove the box for storage in the garage and also because my kid still rides in it. So the holes and the hitch pins and cotter pins provide tool-free securing and removal of the platform and box.

So when I get ready to use the trailer, I hook it up to the bike but leave the frame folded flat. I set the box w/ plywood base down on top of the frame, and line up the holes in the frame and the plywood. Then insert the hitch pins through the holes, and secure the hitch pins in place with cotter pins. Simple!