According to Rashi (Ber 1.20), animals needed a blessing to multiply because humans hunt and exterminate them.
Why do humans need a Bracha to multiply?
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Rav Hirsch on Bereishis 1:28 says that the ברכה for human beings is to enable them to perform the mission that Hashem has given them. It is not just a physical action or set of actions, but the moral and spiritual growth of the free willed human being.
There, in the unfree living creatures, as already remarked thereon, with the giving of the blessing, i.e. with granting the power and ability for reproduction and care of the young, the fulfillment of this blessing is already automatically given. Reproduction and care for the young, in that case are purely physical acts. By the urge which the all-powerful blessing gave, they are carried out with just the same necessity as all other physical developments of living organisms.
Here , in human beings, the blessing, i.e. the granting of the power and ability, is separated from the fulfillment i.e. from using this power and ability for the purposes for which Hashem had intended them. The fulfillment is directed to the human beings who are to carry it out of their own free will as duty. That which in animals is a purely physical act becomes in Man a free willed moral act. But with the fourfold mission, פרו ורבו מלאו את הארץ וכבשה, the whole free willed moral development of the human race is given. פרו is marriage, רבו is the family, מלאו is society, כבשוה is property.