The reason the moon might be a logical part of a Mars expedition is fuel.
There is some inconclusive evidence that the martian poles may have ice within a few feet of the surface. Ice can be melted, then split through electrolysis to form hydrogen and oxygen. The hydrogen and oxygen can be seperately compressed and cooled to make fuel. Additionally, liquid oxygen can be stored as part of supplies for reinvigorating air.
Liquid Hydrogen and Liquid Oxygen are bulky, difficult things to launch into orbit as payload from earth, not to mention expensive to dedicate rockets to.
Since fuel and oxidizer would account for a significant number of the launches, mining them on the moon would be logical.