Well that's easy. He wants all His children to return to him so he made the spaceship earth. A round earth traveling through space would have all the atmosphere blown off as it flew. A funnel shaped earth (where the top was flat) would be able to sustain an atmosphere to live in while we flew through space like a giant comet.
See, that was easy.
Ummm, now can someone tell me what the earth is using for fuel to keep propelling itself through space? It seems that once an object is in motion it tends to stay in motion wouldn't really apply here because eventually we are going to hit something.
You know what, forget about the earth being flat. I want to find the control center that is stearing us around! Where is that? And don't you dare say Antartica because that is where Atlantis is!