As a kid, I just assumed it was aesthetic. Like, someone for an audience of non-musicians to project themselves on to.
As an adult, I recognize that this is almost certainly not the case. Presumably the conductor plays a role that is necessary and helpful to the rest of the orchestra... but I'll be damned if I can't quite figure out what that is. Surely its not just timing? Can't the players just... listen to one another to work that out?
Ok think of it this way.
Let's say that you're a musician on the furthest left of the orchestra in the hall.
You keep rhythm by listening to the musician next to you.
They listen to the musician next to them.
That musician listens to the musician next to them.
This occurs for 50 to 100 other musicians, all the way to the other side of the hall.
You might be in time with the musician next to you but due to micro differences across each of those musicians listening to the musician next to them, you and the musician on the other side of the hall are totally out of sync with one another.
By looking at a conductor, the entire orchestra has 1 single point of reference for timing, instead of hundreds of different points of reference that lead to different sides of the hall being out of time.