But, having played conducted music (double bass), there's another reason. What seems like a fine tuned machine on stage conceals a lot of chaos, and the orchestra may be barely hanging on by a thread. For one thing, at any performance, probably 5 to 15 % of the musicians are substitutes, due to a variety of circumstances. For some commercial gigs, 100%.
Those subs rarely attend more than one or two rehearsals, and sometimes none if it's an emergency. Of course there's a cadre of musicians who specialize in that kind of work. I do it but in typically in a jazz setting. Still, I've experienced looking at some hastily e-mailed PDF's of bass parts, a day before a performance, and saying to myself: "I'm screwed unless the conductor gets me through this."