If you try hitting a horrible fly with something like a swatter without the tiny holes, air doesn’t pass through it, so the fly can feel the air current changing and can fly out of the way.
Pictured: Louie the Fly became a bit of media star when television came to Australia in the late 1950s. He advertised Mortein Fly Spray of course.
The holes on a fly swatter reduce air resistance so you can swing it faster, and it doesn’t press an air current towards the fly, so hopefully he can’t feel it coming.
Therefore you should kill that fly, if you’re quick enough.