The Moon is a staging area and supply of resources. Lunar launch costs are almost negligible to Earth, so large scale mining of rare earths, deuterium, and other metals is viable. And the low gravity means building/launching giant ships capable of reaching the outer solar system is possible.

The research and knowledge gained from a lunar base would almost certainly far exceed the cost of building it as well. Science and practices in medical and botanical fields that would be directly applicable here on Earth, but are not possible or would not be studied without the existence of a lunar base.

If you're looking at future opportunity, figure out how to make a cheap and effective heat shield out of lunar regolith in a vacuum. The major cost of returning materials to Earth is the return vehicle. Having to send parts from Earth very quickly makes everything unprofitable.