If getting 2 people to coordinate is hard -- try 19. Nineteen people who have different ideas of how it should sound (maybe a few without any ideas). While the music lays out exactly what we're each supposed to play, it's not quite enough. Each member must not only have the technical skills to read and play the notes, we all must "hear" it together.
Each horn section has a lead player, and each member of that section must listen and replicate the tone, style, starts, and stops of that lead player. Then each of the lead players must hear each other. The rhythm section provides not only the beat, but hits and kicks and fills to glue us all together.
At rehearsals, we pull out old charts to tighten them up and refine our group skills, then we pull out some new charts to remind ourselves of how far we have to go. Sometimes these new charts are pulled out during a gig to provide the band with a little heart stimulation ("Well, at least we ended together!").
And that's why we rehearse. That, and so our dancers don't sprain an ankle.