Why is a soft item placed inside a plastic bottle?

We drink a lot of drinks daily from cold drinks, whereas gas is loaded. And the bottles of these bottles also you will see that a special type of plastic meter is placed inside their batteries. But sometimes you tried to know why it happens. Let us know. Actually white or any color of the color inside the bottle is a soft plastic, so that gas can not be removed from a gas filled bottle. By doing this, doing this can not be removed from the gas inside the bottle, but if not done After a specific period the gas starts to emit.