Certain controls -- like valves or vent dampers -- are not binary. Even if normal operation is either fully-closed or fully-open, it can technically be anywhere

Certain controls -- like valves or vent dampers -- are not binary. Even if normal operation is either fully-closed or fully-open, it can technically be anywhere in between. Having an absolute position sensor would tell you this, but is complex and expensive compared to having two binary state switches (fully-open vs fully-closed) which is also more expensive than a single state switch. Probably in many cases it just isn't worth it to instrument beyond the single state, especially if the affect of the control is being monitored (eg: flow, temperature, power). However, communicating the state as accurately as possible is important to avoid confusion ("This valve says ON, so that's not the problem" vs "I can see this valve is NOT OFF, but let's double check it's fully-on?").