One encoder can only tell a controller how far the shaft has moved since some reference — and knows nothing about true position until it has been homed. The other tells the controller exactly where the shaft is, in a single instantaneous reading, the moment power comes on.
An incremental encoder and an absolute encoder can look identical from the outside — same shaft, same housing, same electrical connector — but they answer fundamentally different questions. An incremental encoder outputs a stream of pulses as the shaft turns, and a controller keeps a running count of those pulses; that count is a measure of change, relative to wherever counting happened to start, not a measure of true position. An absolute encoder outputs a distinct, unique code for every physical position the shaft can occupy, so reading that code — even before the shaft has moved at all — tells the controller exactly where it is. That difference decides what has to happen every single time the system powers up.