Moses is depicted with a staff because it's integral to his character, as there are several biblical episodes where it plays a central role. When he first tells the pharaoh to let his people go, he proves his divine mandate by throwing his staff on the ground, where it turns into a snake. Later, when the Israelites are wandering the desert, God tells him to touch a rock with his staff to bring forth a spring to drink from. Out of frustration, he hits the rock quite hard; for this outburst, Moses is forbidden from entering the promised land.

Abraham's staff is less central, but since he was a shepherd, he is often depicted with a shepherd's crook.