whether it is called a rudder sensor, rudder sender, rudder feedback unit or RFU, they all do the same task. An RFU measures the position of the rudder, stern drive, outboard or jet drive and transmits this information back to either the autopilot or rudder indication display.
Available as replacements for faulty units or as an upgrade to autopilots that do not currently have a rudder sensor fitted, but rely on software calculations.
If an autopilot relies on software, there will be a small delay in response times while the autopilot calculates the effect the last "blind" movement of the rudder is making. If the autopilot knows exactly where the rudder is, it can make faster, more responsive movements of the rudder. Good examples of when this is important are;
a lighter fin keeled sailing yacht, sailing downwind and surfing down swells will want fast response to prevent broaching
a fishing boat working in rougher seas will want faster response to help compensate for rolling between swells
faster planing and semi-displacement motorboats for finer course holding at speed.