It is a necessary step towards manned missions further out, such as Mars or mining the asteroids.

If we want to do anything extensive on the moon, we will need an orbital platform so remote ops can be done with robots and telepresence.

And it makes sense to establish a testbed for long duration 'deep space' survival before sending folks on year long flights to Mars...

A LEO station is also needed of course, and the deep space gateway should not be all we have.

In short DSG gives us a necessary pre-requisite to putting a human presence on the moon, and gives us the experience needed to go to Mars.