Can someone explain in simple words why we make sure electronics are off before we start up/ shut down our Cessna172 engine?
You shut the components off to protect the sensitive electronics from electrical surges that may occur during start up of the engine.
Circuit breakers are designed to trip open if too much current in amps is drawn from the power source by the electrical components or a short circuit downstream of the circuit breaker. They do not necessarily protect the components from too much amperage and/or voltage generated and pushed through the circuit breaker from upstream of the circuit breaker.
To be safe, disconnecting the components from the power source via a physical switch that can be opened Is prudent to mitigate the risk.
The main problem lies in the behavior of the coils inside the starter. As all inductors, they build up a magnetic field - this stored energy will cause a high spike in voltage while disconnecting. Though it can be dissipated, there is still a slight chance of damaging sensitive electronic devices. And in aeronautics one does not take chances...