Basically it's resource management, both human and materials. I'll leave the human side out, but recommend Gene Kranz book Failure Is Not an Option for more in

Basically it's resource management, both human and materials. I'll leave the human side out, but recommend Gene Kranz book Failure Is Not an Option for more info on why ground controllers are important. It costs a huge amount of money (in fuel) to lift anything up, even just a kilo or two. You're better off lifting only a telemetry system and putting all the processing, display and engineering on the ground. That means having ground controllers to manage it. Of course, in the early days, you couldn't physically fit computers in the capsule. Now we can, but it is still more economical to have most of it on the ground and just use radio to let the systems communicate.