It’s a good idea for smaller rockets. Would be an enormous project to scale it up for heavy-lifters.
Launching small rockets to orbit from a large jet is also a good idea.
If you built a magnetic launcher on the moon you could fling payload packages back to earth...if anything of value was found there.
(The Moon Is A Harsh Mistress)
Supposedly there is a lot of Helium 3 on the Moon...energy.