Considering equal capabilities, airborne missiles are, I believe, cheaper than their land based counterparts (the can be smaller as much of their energy comes from the aircraft), more mobile (easier to fly a plane a few hundred km that drive a massive truck) and harder to destroy ahead of time. Many "smart weapons" are basic "dumb bombs" plus a relatively cheap conversion kit that makes them glide to the right spot.
In Ukraine, I understand, much of the defence against Russian missiles pounding civilians is done from aircraft in the sky, not with rockets from the ground.