Because he knows the preferred insect's habits, life cycles and needs. Perhaps a new artificial diet needs to be produced. The entomologist can assist you in finding such a diet. Also he usually knows what's going on in the field, or he knows how and where to find the required information. To be truly successful at rearing a new product, you may need all the help you can get.
Successful producers have expert help on their staffs.
And so should you eventually. Also keep in mind that you can earn a pretty fair living with just meal worms which are discussed further on. In any case, what I am aiming at in this introductory but very workable book is to give you information you cannot get anywhere else, because it is not available anywhere else. And he had to start out just like you - with one or two insects. Some producers who are quite active still only produce one or two insects.