The pulley is a way of getting force to go around corners, and it's been used to great advantage on pirate ships and for moving pianos. But an example that's closer to home is your knee.
The patella, or knee cap, allows your thigh muscles to lift your lower leg without crushing the knee joint. If the quadriceps were connected directly to your shin, every time it contracted, the bones of the lower leg would grind into the base of the femur, and it would take a lot of effort to move. But by routing the connection over the top of the patella, evolution changed the geometry--when the quad pulls on the lower leg, the force comes from in front of the shin, rather than from directly above, so the shin moves easily.