What is the plastic insert in the neck of liquor bottles for?

On larger bottles of hard liquor, I notice plastic inserts in the top of the bottle that obstruct the opening. Why are they there? Why not on smaller bottles?|||It allows the liquor to be poured out more smoothly instead of splashing all over the place. They allow air into the bottle while you are pouring so that it doesn't "glug" and go all over the place or cause you to pour more than you wanted.|||It slows down the flow out of the neck so you can measure it if you don't have one of the neck inserts that stick out of the bottle.