Many people have answered with the generic benefits of space exploration. But your question is how does SpaceX's being able to land 3 rockets simultaneously benefit anyone?
Reduced cost. Reduced cost to putting up satellites. That is worth billions and billions of dollars simply on the commercial/capitalist side of things.
But reduced cost also is beneficial to science. Being able to get more Hubble Telescopes up, more weather satellites or anything else you can imagine.
And those are just the immediate tangible benefits. You may now fill in the remaining blank with whatever Engineering/Science/Futurism dream you may have. Giant solar panels beaming limitless energy ground-side (meaning nobody will bother sending troops to the Middle East) or whatever other fantasy you might have. Reality is there will be amazing benefits, and we probably haven't thought of them yet.