So just imagine this outlet is in your home, see all that space along the edge. I would use caulking and seal that completely and put that cover back on. Nothing will get by the caulking and that cover, but if you leave it open or not sealed can you imagine what could get in?

When they cut holes in the drywall, they approximate the opening and cut away. These holes if left unsealed allow air and bugs free access into the home. Sealing helps to keep the cold in, the warm out or vice versa and the same with the bugs.  You can seal a lot of these small holes which will help your electric bill and keep those bugs out.