The nature of anything determines its operational capabilities, especially, when left in its natural habitat. For instance, the nature of a bird is to fly, while a fish is to swim. Anything that takes a fish outside water, limits its potential to swim! The believer has a nature that sets them apart from all of creation. It is the nature of dominion, which establishes his mandate on earth.
There're several pillars as it were, on which the believer's mandate is established. The first is to "BE!" The command to be, is suggestive of the fact that one can derail from his original nature. God commands you to be the very essence for which you were created. Don't leave your originality in vain pursuit of what you're not. Be the original, authentic you that leaves no one in doubt that God created you a masterpiece. Your authenticity activates and aggregates all of God's intent for you and makes them function to full capacity.
Then He says, "Be fruitful...!" Fruitlessness displeases God. Jesus, in his displeasure, cursed the fruitless fig tree because it displayed characteristics that were strange and contrary to its nature. Your natural disposition should be fruitfulness in all its ramifications: mind, body and spirit. It is unacceptable to be productive in certain areas and not in others. You should be an all-rounder in fruitfulness that everything about you displays the glory of a fruitful life!