EVERY athlete needs a strong core to maximize their speed and power, and this is ESPECIALLY true when it comes to vertical jump. A strong core helps you transfer from the ground all the way up through your body for maximum height.
Think of it this way - imagine a rocket made of jelly. What would happen if it tried to launch? It would splatter all over, and MAYBE some of it would go upwards. A rocket is made of strong, stiff materials so all of that energy from the blast pushes it straight up. Hollow rockers make your body a rocket body.