No one's really sure; in vertebrate eating amphibs like horned frogs etc they reckon it's for attracting other frogs and lizards (where it's called 'pedal luring'!) like Sam said. Cane toads supposedly do it only when they see other toads. Some other guys (forgot their names) reckon insect eating frogs like darts do it to cause vibrations in the substrate which makes the insects move and more easily seen.