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.
Fundamentally, they’re there to direct tempi, dynamics and feel, as well as to keep as many as 130 individual players in time with each other. If you think they have no proper purpose, try listening to the same piece of music under two or more different conductors. I’d be amazed if you genuinely couldn’t hear a difference.