What is SPD HLD for? | The pushbutton nobody is using | ATR mysteries part 1

ATR 42-600 and 72-600 have a great autopilot. But one pushbutton on the flight guidance control panel is never used. So, why is it even there? Link: Powerplant part 1:    • ATR systems - Powerplant part 1 - General   Attribution: ATR:    • Episode 1: Discover ClearVision™, ATR's in...