The large brackets are used for cabling.  The smaller bracket is used like a lock washer to apply pressure when using the screw to attach the cabling bracket in the cabling channel on the back of the rack on both sides.  The screw is self-tapping and does not need any drilling.  You can use any of the pre-drilled holes in the cabling channel.
I searched for this and couldn't find anything so hopefully it hasn't been answered yet. I just purchased my first PowerEdge 4220 rack and have been happy so far with the whole installation procedure. I did run into a roadblock towards the end of the install manual where it talks about the cable clips. How exactly are you supposed to install these?? I got a bag with the clips, clamps, and screws (look like fat sheet metal screws)...but what exactly do you screw these into?? On page 29 of the manual I can sort of make out the location of where the illustration has them mounted, but there is nothing I can see near those spots to screw into! Am I an idiot? What am I missing here? Am I supposed to drill out holes myself or something? I really want to have these things installed so my cables aren't a big tangle.
PowerEdge Rack Cable Clips