Core fitness is important if you want to be the best player you can be on the field. There's so many aspects of the game where a strong core helps you out. For example, if you want to shield the ball a strong core helps you keep balance. Helps you with a strong shot. It helps you to keep possession of the ball. Helps you to make quick decisions with your upper body, move around. Most of all a strong core mostly prevents you from injuries, for example the lower back.
A strong core is as well essential for a nice header. For being able to receiving the ball with the chest because everything that is, it helps you to balance yourself out, to cushion the ball, it's the core. It's not the leg power itself. It's the core, the abs, the back. It is the core that keeps everything in good shape.