Question

Why does boatman tie his boat to pillar before allowing passenger to step on river bank

Answer 1

every action has equal and opposite reaction as per Newton third law of motion.
now if person off board and move in direction x towards the shore, boat with remaining passengers will move in opposite direction. the rope is tied to arrest this motion

Answer 2

the law of action-reaction states that every action,there is an equal and opposite reaction that acts on different bodies.
In order to jump from the boat and to step on the river bank,Passenger applies force on the boat with his leg in the direction opposite to that of  motion.As a result the boat moves backwards. So, the boatman tie his boat to pillar before passenger to step on river bank.