The car still has to tell the plugs when to fire, and a rotor is one
very reliable way to do so. In a non computer controlled engine a set
of points opening and closing would fire the coil, and the rotor would
route the spark to the proper cylinder. In the newer style, a sensor
detects where the cam is, and then uses that to fire the coil. The roter
just puts the spark to the proper plug. What it does is allow one coil
to be used.
Other styles typically involve several coils, one for each cylinder.
As far as advance goes, it can still be vacuum operated and controlled
by the computer, or a solenoid could advance it. The computer also does
alot more than just fire off the coil, it monitors the air and gas
mixture, monitors the exhaust, and senses when there is a speed change.
It also controls the air and gas mixtures, depending upon what the
sensors tell it.
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