"why do the performers need to be unified? assuming one performer somehow loses the others, can't he just listen to them and adjust? how does the presence of the conductor make this any better?"
The issue is someone has to set the beat, and a large ensemble is large enough that you may not be able to tell that a whole section is dragging from the stage (where the sound hasn't had time to mix). In smaller ensembles this isn't as important, as you can hear the other players and aren't subject to any sound propagation delays. If a section falls just a sliver behind, but not enough to throw everything off, the orchestra will sound "muddy".
"Setting the tempo: isn't the tempo set in advance? don't the performers rehearse and know the tempo?"
Maybe if there is a click track. Moreover, at tempo changes if someone in the bass or rhythm section takes a slower tempo, it is really hard to get the whole ensemble back up to tempo. Bass lines and rhythm have a whole lot of power over the tempo of the orchestra. Moreover, in general, the violins have a tendency to want to rush, and they are seated on the opposite side of the stage from the bass section, for traditional and sound mixing reasons, and absent a "final say" the orchestra may experience a tug of war over the tempo.
Also, as mentioned by NReilingh, live music is live. You're operating without a net. The mood of the audience, the space when filled with people, or some other intangible may require an on the spot change in tempo or dynamics that wasn't rehearsed. Someone who isn't caught up in playing their part, who can see/hear all the voices is the only one who can make the call on the spot to speed up this section, slow down that passage, or start the swell of a crescendo two measures early.
"If most things are known in advance, how much creativity is involved in the conductor's role?"
A lot of what a conductor does happens before the night of the performance. They make all the things "known in advance" during the rehearsals. They chose how to interpret the tempo markings, they inform the balance of the orchestra (Are the violins to quite on the forte, and the drums too loud on the piano)?
Now, the night of the concert, if the work has been done in rehearsal, and everything is ready... A conductor is a human click track/metronome with gentle reminders about what the group should be doing.
That said, even a large ensemble that has played together enough can perform without a conductor... Or in some cases, in spite of the conductor. The Milwaukee Symphony Orchestra was under the direction of McCall during his years just prior to retirement. His motor skills were declining, and his conducting was getting "mushy." The group responded by getting more cohesive. Their next conductor Andreas Delfs was rumored as having a hard time breaking them of the habit of not taking direction now that there was clear direction. The group and he finally stuck a balance and the MSO grew in acclaim under his tenure.
"or in other words: what makes a conductor a good conductor?"
A good conductor is one that is able to communicate/cast their artistic vision, who is able to get an ensemble to play well together, and challenge/push the players to improve (both together and separately). They must also be forgiving, and human. They must allow some artistic freedom for the musicians, but not so much that the group cannot work together.
I like what Benjamin Zander has to say on the subject, "The success of a conductor is not measured in tickets or CDs sold, but in the shining eyes of the people around them." Those eyes are not just the audience but also the musicians. And he extrapolates that we should all ask ourselves this question of the people around us (friends, family, etc.), "Who am I being that their eyes are not shining?"
Personal example, one conductor I had (in high school) wouldn't let us play the final note of Beethoven's 5th in rehearsal. Each time we got there he would cut us off in the final rest, and have us start the movement over again (or from a passage he thought needed more work). It was crazy making, maddening at the time. Come the concert, when the final note was struck we hit it with all we had because we hadn't gotten that closure of everyone playing that cadence together in rehearsal. The moment sent chills down everyone's back.