The conductor as part of the rehearsal is always overlooked here. Music, especially ensemble music with 80+ muscians, can easily be interpreted in many ways. Even when the sheet music has guidance, there are always degrees to decide on. How fast to go here, what volume to play here, on and on, and generally every instrument has different requirements to make a piece fit together. Someone guiding that process, having an understanding or at least a point of view, is a big part of the job. The stick wagging is just the end point after all the other work is done, but it in itself is also useful for execution and consistency.