I was told many years ago by an orchestra leader in New York City that the most important part of any band or orchestra is the drummer. “The drummer,” he said, “is the steam in the engine. It is the drummer who sets the beat and everything else flows from that rhythm. Without a good drummer, you have no direction, no impulse leading you forward.”
The lesson I took away from that conversation is that the beat the drummer sets is what forces everyone into the same context. Without that anticipatory thump, everyone else makes their own time, and you end up with chaos.
A few years ago, I remember something Late Night with David Letterman Sadowsky bassist Will Lee said in an interview about how having a bad drummer can sink a session, and how he then has to step up as the bassist and take over the role of the drummer.