To take the cake analogy further: the recipe didn't magically fall from heaven. It's not the one correct way to make a cake. It's a method that someone else has tried and tested and has decided gives a pleasant result. There will be other recipes using the same ingredients which will give a range of different but also pleasant results.All development is post exposure 'something' but I don't think manipulation is the correct word. If it's a standard process, I don't see how it can be a manipulation any more than following a recipe to make a cake is.
C41 film sent to a lab for development and printing by a standard process could just about be described as manipulation as the printing stage adds a bit of automation but it's not usually in the photographer's control.
Steve.
It's the same with film development. You could just follow someone else's recipe (use their methods of manipulating the exposure). Or you could experiment yourself. Or you could hand it to someone else to develop for you. You'd likely get different (but acceptable) results in each case.