What is the purpose of a conductor?

Firstly, what does this have to do with law and ethics?

But since you ask, the real work, as I know from being a choral singer, is all done in rehearsal.  There are directions in the score, but the conductor will have his or her own interpretation of tempo and volume and will have taught that.

Some pieces are at an absolutely regular tempo and same volume throughout.  Think of church hymns.  They're always written like that.  But even so, it's going to be a mess without a conductor - in this case, even with everyone knowing the tune, the organist is in charge and will do a playover to set the tempo and so everyone knows when to start.  Mind you, that means the organist can ruin it.  I've been at a service where Mrs Organist slowed down all the time - no, she shouldn't have listened to the congregation, SHE is supposed to lead US!  OMG this was an important ordination and I could see one of the ministers, who has written hymns, looking up at her with "wind it up!" gestures.  I felt for him.  It didn't help that the tune was Bryn Calfaria - one of those Welsh hymn tunes in a minor key.  The usual upbeat St Helen for this hymn would have done it better.

How about unaccompanied hymns?  I'm a British ex-Methodist and this is normal at District Synods and the national Conference - in that case, there is a precentor with a tuning fork who is effectively a conductor to set the pitch and pace.  (It's great for waking everybody up in all-day meetings - every couple of hours, get everyone to their feet and have a good sing.)

For a piece like that, probably the orchestra could even manage without the boss on the night, he's talked it all through in rehearsal, but even then, who's going to give the signal to all start and end together?

For something more complex (I can think of almost any choral work by Parry as he loved to pull about with the tempo a lot (have you ever heard "Blest Pair of Sirens"?)), again all the real work is done in rehearsal but there's going to be even more of that and it really helps to see the same firm direction again during the performance.

Yes they are like movie directors - OK, there aren't sets and lighting etc to arrange, but there's still a lot of work in instructing the orchestra beforehand so it comes out the way he wants it.  Like a movie, you only hear the final result.

No, most aren't composers, but that's an entirely different skill.  Just the same as mostly, movie directors didn't write the script.

At a very basic level:  musical directions in the score are extremely vague - fast, slow, very fast, loud, quite loud, soft etc etc.  How loud is "loud"?  How slow is "slow"?  In a small ensemble like a string quartet it is relatively easy to gain consensus and control the performance - hence no conductor for a string quartet, though 1st violin generally takes on the role of leader.  But in a large orchestra every player will have a slightly different idea of tempo and volume so the conductor  decides and controls the performance.

The conductor also applies many nuances to a performance 

Conductors are the most important person on the stage...ask any performer. They can make or break an ensemble. They communicate using their body, hands and eyes to get what they want from the piece of music.

Orchestral works can have many changes in tempo and volume. Unless you can find 80 orchestral musicians who can read each others minds perfectly, you need a conductor. Conductors also cue soloists. A conductor can be an orchestra's music director, chief publicist and fund raiser, and make hiring and firing decisions. Conductors can also be composers and soloists, like Leonard Bernstein.