Yes, that's close to what I do.
You have wires everywhere and hot fragile light bulbs falling off and breaking and shorting and melting stuff, and it's a stupid, time-consuming mess.
I want one nice neat box with a switch on it and banana jacks in it, and a voltmeter and a light, and maybe a squeaker speaker. And perhaps an ammeter? Maybe some diodes and fuses for protection. Probably some other stuff better minds than mine can think of. And I don't want to cobble together my own.
Someone above couldn't figure out why variable loads made a difference. As one example, let's talk about testing a 12VDC relay. The usual failure mode for relays is that the contacts inside get pitted over time, and become intermittent, resistance builds, and finally the contacts are too burned to work. But until they fail completely, you'll still get a good voltage reading even though the contacts can't pass enough current to do much. You can also get a relay that will deliver an amp or two OK, but not more than that.
If I can load a, say, 40 amp rated relay with 5 amps, and then poke, tap, jiggle, and feel it, I can quickly tell by the sound of the squeaker or the brightness of the light, or by feeling the relay for heat (or scanning for IR if I'm feeling fancy), that it might be getting intermittent. At one amp it would probably work just fine and would not generate much heat.
In troubleshooting motorcycles, I've encountered many examples of pinched, corroded, or internally damaged wires and connectors that would pass a voltage test, or even pass an amp or two for a test light, but could not deliver, say, the three or five amps for a coil or headlight, or they'd start getting hot only at higher loads.
Same for many switches -- for example, horn, starter, and light switches. The current they can pass, and the resistance in the switch, is important.
