With car audio, you are better off knowing what kind of voltage your amp is seeing, than what the alt is putting out. It does no good to know your alt is putting out 14v, but your amp is probably getting 12.8v because you have a bad wire.
This is where having dual voltage gauges helps. You can read the output of your alt, and the input to the amps. Can help to spot wiring troubles.
With multi-amp systems, you can monitor at the closest distribution block that serves all amps.