Why is there an opening where the burner connects to the gas line on a gas oven?
(based on comments)

The opening is called a Primary Air Mixture. 

Gas flowing through the line from the service point passes this opening, picks up air (oxygen), mixes and is ignited at the burner head.

Pressure in the line keeps the gas flowing. As there should not be air (oxygen) in the gas lines, this port allows the proper amount of oxygen to mix into the gas allowing ignition. Most gases (all?) require oxygen to burn.