When you say "ranco" in the fish world you are talking about a Ranco controller fitted with a plug and receptacle. Basically you plug them in, and plug whatever you want to control into them - usually a chiller or a heater.
They are incredibly versatile gadgets... a few applications...
- Controlling a chiller...
- Controlling a heater that has no thermostat built in
- Controlling one or more heaters with thermostats as a redundant backup
- Hooked up to your lights to cut the lights off if the tank's temperature rises too high...
In this case, I'm talking about hooking one up to your heaters. In an example, you might set your heater's built in thermostats to 82 degrees, plugged into a Ranco set to 78. The Ranco would control the temperature all the time, but should it fail, and the tank's temperature rises to 82, the heater's builtin thermostats would open up at this point, stopping things. Or, you could do the opposite, with the heater's thermostats doing the day to day work and the ranco being a "high temperature cutout."
I have yet to find a heater that I 100% trust the builtin thermostat on. One failure of one can cost me so much money... it's worth it. That said, Rancos are not cheap. However, they are industrial grade devices designed for maximum reliability. You can't say that about just about any aquarium heater.
But Ranco is a brand name of thermostatic controls that are used all over the place, so if you googled, that might be where you saw non-aquarium usage.