I think to diffract the incoming sound waves instead of bouncing them back to the capsule which would cause comb filtering effects.
There is a thread somewhere here where a user modded his TLM49 with a dome and it made a noticeable improvement.
Thank you, I also assume this to be true. Indeed, I am interested in other's opinions with regard to effects they have heard and the original concepts manufacturers drew from to arrive at such designs that utilized the dome.