IMO, the ammmeter gives an indication of what's going ON with the charging circuit, so you'll know if the alternator is doing its job. If the ammmeter is always on the neg. side, you know you're either drawing too much, or the alternator isn't putting out enough, and your battery is going down. Then your engine will eventually quit. The voltmeter gives you an indication of how 'full' the battery is. Is the voltage isn't high enough, the engine will quit, or the starter won't turn the engine over. You'll notice that it's the older cars, trucks, tractors, etc., that have the ammeters,because ammeters were used when they only had generators on the engines instead of alternators. Even the generators weren't that reliable/efficient, so you needed to know if it had enough RPMs to put out the voltage to charge the battery. Contemporary alternators are relatively much more reliable, so battery voltage became more important, though not necessary, so they even got rid of that! Instead we now use the indicator lights. Again, it's a metter of preference.