Anti Gravity Hats was a product sold at Weasleys' Wizard Wheezes in Diagon Alley. A slogan written on the package read "Ruin a gentleman's day by making his hat fly away!", so, presumbly, the hats are supposed to be used as a prank. The Anti-Gravity Hat trick has been around for generations in the Wizarding world, and they are not to be used on Muggles.