Part core concept, part outdated nonsense that's taught due to tradition.

If you've got components that can halve a voltage, then by putting that in a feedback loop you can double a voltage.

And you can make an accurate amplifier even if some of your components - like the op-amp's gain - are inaccurate. So long as your voltage-halving components are accurate, your voltage doubler will be accurate whether your op-amp's gain is 10000x or 20000x

You can chuck other components into the feedback loop too - want a higher current output from your voltage doubler? Have the op-amp control a high-power transistor.

You know how a feedback loop can turn a voltage-halver into a voltage-doubler? It can invert other mathematical functions too. Put a capacitor into your op-amp circuit and you can integrator or differentiate. There are even op-amp circuits for summing inputs!

You now understand feedback loops, precise gains, power output stages, integration and differentiation. You can now make a PID controller - a key concept in control theory! Just what you need to position control a robot's joints.

Except making PID controllers out of op-amps is obsolete; they're all done in software these days.