Adding your preferred primary profession(s) helps you get discovered in the industry. It also depicts what you want your known for profession to be on your page in IMDb and IMDbPro, so decision makers in the industry can find you and reach out to you or your reps for potential opportunities or your fans can learn about your preferred profession.
Your primary profession selection orders your filmography display in IMDb (desktop) accordingly, thereby displaying your credits based on your primary professions first followed by all other professions.