Leonard Mansky quoted Rambam saying that it is a mistake to think that the whole world was created for human beings' sake. Therefore, we cannot know G-d's purpose in creating the world. But since G-d is perfect, all His actions are perfect, and partial goodness would not be enough. So the purpose of Creation is not just our existence, but our attachment to G-d. The purpose of the physical world is not just to make our existence possible, but to make it possible for us to attach ourselves to G-d and to fulfill His will.
Every tool has an immediate purpose and an ultimate purpose.
For example, the immediate purpose of an alarm clock is to ring at a certain time. The ultimate purpose of an alarm clock is to wake us up. In the same way, creation has an immediate purpose and an ultimate purpose. Rambam is talking about the ultimate purpose in his argument. Those who say that the world was created for our sake are talking about the immediate purpose.