Well many festivals are based in towns and use local hostelries.

To buy a drink may require you to 'buy' a glass (i.e. put down a £1 deposit or similar) or alternatively drink out of some flimsy plasic container that spills half your drink when you attempt to pick it up.

It has also been known for pubs to run out of clean glasses from time to time.

As sessions are also held in these pubs you may wish to move from one to another venue without necessarily quoffing your ale in one go; a tankard enables you to take your drink with you.

Maybe you just have to be there :)