Because sometimes you use child inputs which, again, contain their own colour, displacement, etc and the colour input wouldn't be necessary, so you would uncheck it unless you wanted a colour to paint everything else that child inputs doesn't cover (like the child input is masked).
The colour checkbox inherently should be checked, if you want colour. Otherwise if you don't, you uncheck it. If you delete a node it won't automatically uncheck, but if you drag a input away from the colour input it will automatically be unchecked.
The colour input doesn't overwrite colour or input of the child. The child is essentially above the surface layer in rendering order.