Easy. We all have had the "old Mopar" light dimming at idle. Voltage can run all the way from 12V to 14V and even more. This is the same thing as your home power going from 120V to 140V. The guages would have gone nuts if they would have been connected direct to "varying" 12V power.
Frankly, I'm suprised these things worked as well as they did. I'm not sure who-all used this system, certainly Chrysler, Ford, and I think AMC. IHC may have, as well. I don't know about Desoto/Kaiser/Stude and some of the others.
I can still remember about 1967 or so, when the gauges in my '60 Ferd Falcoon went completely nuts. I went into the parts store where my Father worked, and he didn't even have to look it up. "Probably this thing" he said.