Why some robots have to run a homing routine after a power loss — and others just power up already knowing exactly where every joint is.
Both devices bolt onto the same spot on a motor shaft and both report shaft position back to a controller — but they answer a fundamentally different question. An incremental encoder reports how much the shaft just moved, as a stream of pulses the controller has to count. An absolute encoder reports exactly where the shaft is right now, as a unique code, no counting or history required. That single difference is the entire reason some machines need to "go find home" every time they power up, and others don't.