Ok, so you're at a concert and they are selling drinks inside. But, they won't let you in with yours because then they won't make that money off of you. Plus, sometimes they don't have what you want inside.
Problem solved NOW! You can take your drink with you and if they pat you down, say, "Hey, that's my beer gut! Go laugh at something else!"
I said it was the best invention known to man. Now, was I wrong?
What can you put in it?
Technically, anything you want. Any beer, liquor or mixer. You can even put baby formula in it. But, I'm not sure what the purpose would be.
Are they illegal?
There is no law that states you can't take alcohol in where they sell alcohol in the first place. But, the vendor might have a rule against it.
What is the purpose of using it?
Mainly, it helps save you money on drinks in a sporting or such other event. But also, it helps you enjoy yourself more because you can have the drink of your choice rather than what they have.