I have a chest freezer with a thermostat dial on the outside of the unit, so why do I need another temperature controller? What's wrong with taping and insulating a wired digital thermometer to the carboy and adjusting the temperature with the freezer thermostat based on the digital thermometer? I can't find any posts that address this. All I keep reading is that you need an external controller
The main reason you need an external temperature controller is because the thermostat on a fridge or freezer isn't in the right temperature range. Also they are typically not calibrated and offer only approximate control.
Some appliances have a digital thermostat, and you might be able to use these if they fall within the required temperature range. For example, I have a fridge that will go up to 9C/48F, which is fine for fermenting lagers. A diacetyl rest at 66F/17C is out of the temperature range, so I did that by moving the beer to a room at that temperature.
Having a temperature controller allows you to target any temperature range independently of the fridge thermostat.