Pulleys are often used as parts of compound machines. They function as helper machines, easing the lifting power. In fact, a pulley system can cut the force necessary to lift a very heavy object in two, four, etc., depending on the designed system. With these pulleys you can study and measure the force and distance trade off with compound machines. These pulleys are good for science experiments as well as other functional tasks.

This compound machine uses three pulleys to lift a heavy weight with ease.  This particular design will reduce the lifting force from the actual weight of the object by about 30 times. Of course, the actual design will determine the force necessary because of all the parts involved.  

You are buying a set of five pulleys only. This picture is a sample and illustrates the functions of pulleys.