So that people like you can complain about it on the internet.

It takes a lot less to launch a rocket from the lunar surface to anywhere in the solar system than from earth. If we ever get manufacturing working on or around the moon then it opens up a lot of possibilities for larger payloads. This means we can send more further and it makes the whole process cheaper.

Because it's there.

There are loads of reasons but if you want a practical one we should do it for the helium 3 and other materials that can be found on the moon and in space.