>Ideally, you want the pressure at exit to be equal to ambient pressure. (This is mainly about efficiency?)

Yes, this is ideal. I'll expand on this a bit. Take a look at this image of possible nozzle flows. The first one is underexpanded (as in the nozzle didn't expand enough, so the flow is higher pressure than ambient), the second is perfectly expanded, and the third is overexpanded.

Let's go back to Newtonian physics. Remember how every action has an equal and opposite reaction? This is how rockets work - gas is pushed out the back, and that pushes the rocket forward. However, if the nozzle is not perfectly expanded, then that means the gasses are moving out at an angle - either radially outwards (underexpanded) or inwards (overexpanded). So if that gas is moving away from the rocket, there must be an equal and opposite reaction in the same direction - if some gas is moving up from the rocket, then the gas must have pushed it down. Since it's a circle, it cancels out, but this is the primary issue with exit pressure not at ambient - some of the energy coming out of your rocket is being futilely used to push on the walls of the nozzle rather than to push the rocket forward. With a perfectly expanded nozzle you don't have that problem, as all of the momentum is pushing straight up the rocket's velocity vector.