Fundamentally they're there to keep everyone in the orchestra in time. So for something like 'The Flight of the Bumble Bee' or 'The William Tell Overture' you'd need a lightning conductor. For something more sedate like the largo from 'The New World Symphony' you could probably get away with a semi-conductor.
There is probably no truth to the rumour that Sir Simon Rattle was the inspiration for the original wacky waving inflatable arm flailing tube man.