When God created Adam and Eve, He commanded them to be fruitful and multiply. God wanted them to fill the earth with people. They were the only humans to do this.
One day, Jesus would be born from one of their descendants.
Webster's Dictionary says the following:
fruitful \Fruit"ful\, a. Full of fruit; producing fruit abundantly; bearing results; prolific; fertile; liberal; bountiful; as, a fruitful tree, or season, or soil; a fruitful wife. -- Fruit"ful*ly, adv. -- Fruit"ful*ness, n.
Be fruitful and multiply and replenish the earth. --Gen. i. 28.
[Nature] By disburdening grows More fruitful. --Milton.
The great fruitfulness of the poet's fancy. --Addison.