As we know there's only one ball for all play on the field. So there are many situations where we actually do not have the ball but still have to be at least mentally part of the game.
That means we have to get ready for the ball. We have to, for example, the defenders have to chase down the strikers. They have to close gaps. They have to move forward. They have to move backwards. They have to attack, shield the ball, keep the striker out of line of the goal to make sure he or she is not able to score.
This all happens without the ball but there's a wide range of movements that we have to be aware of without focusing too much on moving around itself. Because we have to keep an eye on the ball. We have to keep an eye on the opponent. And focusing on moving around. There should not be one thing that we have been distracted by.
As a striker, for example, you want to get rid of your defender. You move sideways, you move backwards. You have pressure situation in the back and still you have to be able to, for example, receive the ball while when it's finally coming in.
To get ready, to be in the ready position, you have to make sure that you're staying on your toes. You're moving sideways without any struggles. You're moving backwards without any struggles. There's nothing that can distract you from the ball that is coming in soon.
And to keep your head off those basic things, soccer agility as well as quickness and fitness is essential for the game.