It's a question of volume in the intake path...

No air will get into the throttle body until the volume is filled up. Doing that will take time. By removing the silencers and plugging the resulting holes, you're reducing the volume thus reducing the time it takes the remaining volume to be filled thus improving throttle response.

There was a big discussion a while ago about this where the real physics this was talked about. I gotta figure that this is the most popular sub-questions of of this topic.