These are perfect for those times you don't want to use an airlock but you also don't want the hole in your stopper letting all kinds of bacteria into your beer. Just sanitize one of these little guys and stick it in the hole!
I use these to make a solid stopper when I want to aerate my wort by shaking the carboy. But more importantly, it's super cheap. So grab a few and throw them with your gear. At some point it will serve a purpose.
So they made a stopper for your stopper! Goes in the stopper hole easily with a nice proper fit. It will create an airtight seal, so the slightest buildup of pressure can cause the big stopper to pop out. It works well when you don't have a sanitized airlock handy.