Because it would be considerably easier to launch deeper space missions from a Moon location than it would from Earth.
That's fine if you're planning deep space missions. A permanent moon base would be expensive right now as well as inherently dangerous.
As for mining the moon, that's all very well if you have a cheap and reliable way of getting the mined material to Earth. Which we don't. The only thing that might be worth mining is Helium-3, which is still extremely rare on the moon. Not as rare as on Earth, but still extremely rare. You'd need mining on a massive scale, processing billions of tonnes of moon rock per year, to provide the fuel for helium-3 nuclear fusion power plants...which don't exist anyway.