This is how I've understood the aircraft: "aircraft designed solely for close air support, including attacking tanks, armored vehicles, and other ground targets."
Essentially, a prime mission objective is to loiter over an area and provide air support. Something most fast-moving aircraft are not exactly made to do. If a request comes in to target a tank; the plane kills the tank. If a request comes in to target a truck; the plane shreds the truck, and whatever happens to be inside, to shreds. If a request comes in to strafe a defensive position; then the plane puts big holes in those defenses. As for personnel, the plane may not kill them but it sure as heck puts a good scare into them. Stories indicate friendly soldiers like seeing, and hearing, the plane in the area during a firefight. It has more uses than the bullets it fires.
Also, keep in mind that the A-10 can carry other weapons than just the cannon it's built around.
Now, is it expensive in today's world for that purpose? Probably so.
Apaches are nice too. I'm sure one day someone will consider them an expensive disaster as well.
No, it was designed to kill tanks, not personnel. Its 30mm cannon is complete overkill for that role.
Apaches are MUCH better for ECAS, and GPS guided bombs are fine for the general CAS role (with troops cuing the targets by laser cartography).