1) The things we learn from a lunar base will make it easier to explore further into the universe.
2) There may be many military advantages to having a manned outpost on the moon. Some of these may be known, some may not be known since technology hasn't gotten to that point yet. But when technology catches up, it would be wise to have a moon base to deploy it.
3) If we don't do it, the Red Chinese certainly will.
4) There may be raw elements located beneath the surface of the moon that could prove advantageous to whichever country owns them.
5) As trips to the moon base increase in frequency and therefore become cheaper, there will likely be commercial/private interests looking to pay for a trip to the moon.
The moon is useless, Mulder!
There won't be a USA in 2200 unless we do.