why do idiots at the fairground shout roll up roll up?

This is neither a joke or a riddle but a showbiz question

They are called Barkers who try to entice you to their stall to spend your money This tradition goes back hundred's of years and was passed on throw the family Hope this helps

Do not call people idiots. The saying has been used for hundreds of years and is traditional. It is a way to get heard.