It's good to be available when others need you.
James works at a school and the school asks him to pick up a large order of groceries from a distant city.
James wants to give them an answer.
James agrees and clears out his schedule, so he can pick up the food.
James says that he is busy and declines to get the food for the school.
The school gets the food that they ordered, and they don't need to sacrifice someone else to do it.
The school needs to find someone else to get the order and that person can't do what they need to.