(a) to change the direction of force,
(b) to obtain the gain in speed.
Solutions:
(a) To change the direction of force: a single fixed pulley is used to lift a bucket full of water from the well by providing downward force instead of upward force when the bucket is hoisted up without the use of a pulley.
(b) To gain speed: when cutting cloth with a pair of scissors, the blades of the scissors travel longer on the cloth while the handles move a little.