My favorite conductor joke:

What's the difference between an orchestra and a bull?

On a bull, the horns are in front and the asshole's in the back.

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I took a conducting class when I was a music student, and as a kid who played in rock bands, the only frame of reference I had for coordinating an ensemble was maybe 3-5 people playing a song together? So it was easy for me to think, "Well, if everyone knows the music front and back, they're in tune, and they start and end at the same time, what do they need this guy for?" But when you imagine an orchestra of 70-100 players who are meant to sound like one big well-oiled machine, it does become necessary to have a central figure dictating tempo, balance, dynamics and overall feel.