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?
If everyone is positioned close together, and they do all their practicing in the same location where they will eventually perform, and they are very well used to each other, and the piece/movement both starts out and ends with just one instrument or voice, and the piece is continuous without any pauses longer than one bar, and there are no fermatas or accelerandos or removedandos, then maybe you can go without a conductor.
Every time you need to start, stop, speed up, slow down, sometimes time change, sometimes volume change, and sometimes for solos, there is a need for coordination.
Reversed accelerandos