Uh oh. I'm disagreeing with Carl Froehlich on an electrical issue. This can only go bad for me.
I think I'm going to join you about disagreeing with Carl, mostly. An ammeter may not be required, but it certainly is good to have.
Let me tell you of an incident where the ammeter was indispensable in isolating a problem. I have no engine-driven fuel pump, but rather dual electric pumps. Those dual electric pumps nominally draw about 5-5.5 amps of power each during normal use. I have them individually breakered at 10 amps each.
One day I was cranking up to go play in the wild blue yonder, and before I started to taxi my engine died - just up and quit, cold - and in my headset the Dynon Skyview is warning me of low fuel pressure as the prop spins down. I had one of the two pumps turned on at the time, the switch was still on, the fuel pressure was zero - then I saw the breaker had tripped. Hmmm, ok, that's odd. Reset the breaker, try the same pump again, fuel pressure comes up, restart the engine, watch it for a bit - BAM the breaker trips again.
Ok, simple enough, I've got a bad pump, right? I've still got one other pump but I'm not flying on just one. I'm grounded until I solve the problem. I reset the breaker but switch the other pump, crank up again, and run for about 2 minutes when THAT breaker trips. Hmmm - two bad pumps? What are the odds? Something else is going on here. Possibly a breaker issue? Voltage was completely normal for all this.
So sitting there without cranking the engine again, I looked at my ammeter while running one pump, then the other. I saw a delta of 9 amps on one side, 10 amps on the other, and tripped the breaker again. The pumps were BOTH pulling nearly twice their normal amperage, the breakers were fine. Again, the odds of losing BOTH pumps at the same time are quite long, but the ammeter confirmed they were both pulling too much current, and the fuel pressure was bang-on normal at 44 psi the whole time (sensor is FWF at the injector rail). What gives?
At that time I had the dual output of the two pumps Tee'd together into a single output, then a final filter assembly before sending the fuel FWF. Turns out I had gotten a load of what looked like mud-dauber nest (mud) into my left fuel tank, which was fine enough to make it through the pre-filter and pump but plugged that final fuel filter, causing both pumps to push against a much larger than normal backpressure, drawing more amps and finally tripping their breakers. (The mud originated from my fuel tank/pump in the hangar, I found the remains of the nest inside the fueling nozzle.)
So knowing the data above, I deduced the fuel filter blockage and it became a very simple fix - but I want you to think for a minute about how many hours, and how many dollars spent replacing/checking components, were saved by the presence of the ammeter in that case. Would that total cost offset the value of installing the ammeter, do you think?
Which parts would you replace first - the pumps? Then the breakers? And when the problem persisted, what next?