When you're home and moving about the home, but want your alarm on. The motion detector(s) are OFF, but the entry delay is ON.
When you're heading to bed and are not expecting anyone to enter the house. This mode allows movement throughout the house because the motion detector(s) are OFF and the entry delay is OFF (i.e., alarm will sound immediately if an entry door/window is opened).
When you're leaving the home and no one will be at home. The motion detector(s) are ON and the entry delay is ON.