This will bring out a bunch of dissenting opinions but here is my opinion anyway.
A voltmeter will tell you what you already know. If you're driving down the road and you look at the voltmeter, it will say 14 volts. You already knew that. The heater is working, the radio is playing and the engine is running.
Tomorrow you get in your truck and "click" or maybe nothing. You look at the voltmeter, but you already knew it wasn't going to say 12 volts.
Every day you drive down the road and look at an ammeter, it says "0" or slightly above. Life is good. Tomorrow you look at it and it says 5 or maybe 10 amps. You don't check it out today. The next day you put a voltmeter on it and it says 14 volts. You find the receipt and it says this Diehard is only six years old. What the heck? Tomorrow you stop and get a new battery. Life is good again.
A long story, short. An ammeter will give you a warning, if you watch it.
