Second, the orchestra is not necessarily the idyll of unity that it appears to be. It's impossible to herd 100 cats together for every performance, and the typical contract allows players to take nights off. They have to hire subs, and for some performances, throw an entire orchestra together for an occasion. Depending on the level of the orchestra, there may be one or two players who are quite proficient, but not quite up to speed on the repertoire. Those folks may need the conductor to get them through tricky portions of the pieces, or to address issues of interpretation on the spot. I've had my arse saved by a perceptive conductor on more than one occasion.