The only way to add herbs and spices is "to taste."

One of the goals in the restaurant business is to produce the same dish the same way on successive occasions. Given that different line cooks are tasked on different occasions to prepare the same dish, one way that a chef can be certain of getting the same product -- despite the varying personal tastes of the line cooks -- is to insist on the measurement of spices.

I didn't say that it is the best way, nor the way that is preferable in the home kitchen, but it is pretty common in the restaurant trade.