Someone has to make the musical decisions. In a string quartet, it might be the first violin. In a baroque ensemble, as the article stated, it might be the harpsichordist. In a symphony, it's the conductor.
Sure, a conductor keeps time, but not so much during the performance itself. With enough practice, any professional orchestra should be able to keep time on its own, but during rehearsal, someone still needs to say exactly what that tempo is in the first place.
Having played with many amateur orchestras, currently rehearsing with one weekly, I've seen good and bad conductors. Just like a CEO or basketball coach or political leader, the conductor shares his/her vision of "success" yet knows the low-level technical details of the craft required to lead them there. The part about waving one's arms on stage has little to do with it.
An organization -- any organization -- acquires the characteristics of its leader. Therefore, one must not underestimate the importance of a good conductor.