Ballerinas need balance in order to perform their routines. Ballerinas wear specific ballet shoes that have a flat toe on top. Then the ribbons are wound around the leg and tied to keep the shoes on. Since ballerinas must stand on their toes during the majority of their routines, they need to be able to balance. Sometimes, ballerinas are thrown into the air and then must fall to the ground.
They also must spin around on their toe. All of these steps require the ballerinas to maintain their balance and not fall down. Therefore, they must have a strong body and especially a strong pair of legs. Then the ballerina can remain on their toes while moving the rest of their body around.