Why do I need call onChange and onTouch in writeValue when implementing ControlValueAccessor in Angular?

Read this article that explains the ControlValueAccessor in great details:

• Never again be confused when implementing ControlValueAccessor in Angular forms

You usually need to implement ControlValueAcceessor interface on a component if it's supposed to be used as part of an Angular form.

I commented out the second and third line of writeValue(...) method and, as far I can tell, nothing broke.

This is probably because you're not applying any form directive - formControl or ngModel that links a FormControl to your custom input component. The FormControl uses writeValue method of the InputComponent for communication.

Here is the picture from the article I referenced above:

The writeValue method is used by formControl to set value to the native form control. The registerOnChange method is used by formControl to register a callback that is expected to be triggered every time the native form control is updated. The registerOnTouched method is used to indicate that a user interacted with a control.

Why is it important to execute the call to onChange(...) and onTouch(...) in writeValue(...)? What will go wrong and under what circumstances can it be expected?

This is the mechanism by which you custom control that implements ControlValueAcceessor notifies the Angular's FormControl that a value in the input has changed or the user interacted with the control.

...discovered that I couldn't tell anything going bananas when I removed setDisabledState(...)...Does it really need to be implemented?

As specified in the interface this function is called by the forms API when the control status changes to or from "DISABLED". Depending on the value, it should enable or disable the appropriate DOM element. You need to implement it if you want to be notified whenever the status of an associated FormControl becomes disabled and then you can perform some custom logic (for example, disable your input component).