reason to go to the moon is to establish a presence there. For one we would not be launching a Mars mission from the surface of the Earth...too costly in fuel. This would be launched from either the ISS or from a moon facility.
Two, there is very little atmosphere on the moon, using a telescope on the moon would have resolutions very similar to that of Hubble, even better if you used a larger telescope or a system of telescopes. The reason the Hubble is so great is because there is no atmosphere degrading its visibility.
Three, with the reduced gravity of the moon future space flight will most likely occur from there. We could put the craft together on the moon from pieces either made from a moon base or from the Earth, and then launch it from the moon thereby saving on fuel and allowing for larger vessels.