There are quite a few reasons to build a habitat on the moon, however IMO, they are all directly related to manned missions. I do not believe there is anything available on the moon that's not available on Earth.
I believe if we do start sending manned missions to mars, the moon would be the best stepping stone to get us there. If for no other reason that to place a refinery on the moon to extract fuel. The problem is, we have to get into the mindset that we need people up there. Personally, I believe we do because, well we have people everywhere else we can get to.
Here is the way I would like to see us move forward.
First we need to make the decision that we want a permanent human presence on the moon. Who cares if we have to rotate them out every three months or so, we just need to make the decision.
Next, we develop the habitat. Without getting into a bunch of specifics, I believe we can develop a habitat that could be used on mars, on the moon or just in space, kind of like a tent can be used in a forest, in a desert or even hanging off the side of a mountain or in the arctic. One, multi purpose design that can be deployed whereever we need it.
Add some "Warehouse Pods" to ISS. This would allow us to launch several unmanned flights to rally the cargo to a single point in space. This would serve as a staging area for the initial moon flight. The CEV/LEM unit would be assembled at the ISS staging area. The crew would fly up seperately for the mission.
Once everything is assembled in LEO, the crew hops in for a burn to the moon. Once in a lunar orbit, first, the cargo pods are dropped to the colony site. Once the pods are down, the crew will land on the site and begin assembly.
The next step would be to insert a staging area in lunar orbit, similar to using ISS as a staging area. This way, you can have craft whose sole job is transport people or cargo from space station to space station. this way, the only launch vehicles needed would be from earth to LEO or Moon to LLO.
Personally, I believe this is the most cost efficient way of colonizing the moon and eventually, mars.
And like someone said here,it will be expensive for only one country or one company to do this.
Next manned mission to the moon and eventually Mars will be an international operation,just hope Im around to whatch it.