There is a distinction between formal and informal writing. Personal pronouns are considered informal as you would write a letter to a friend. Academic writing is supposed to be formal. It's like holding your pinkie finger up when you drink a cup of tea. Nobody has any idea what the purpose is but that's how it's "supposed to be" done.
The choice of words that gives the reader a clear mental picture makes the best communication. However, if the purpose of the writing is to get the best grade, "supposed to be" always wins.