An important message is given in the Bhagavad Purana in which Vishnu as Gopala is not merely one who looks after cow but also the Earth. It is said that Mother Goddess Earth takes the form of Cow and complains to Brahma about the cruelty and injustice committed by kings and greedy men on earth. Brahma then takes her to Vishnu who promises to get the earth rid of Adharma.
Srihari Vishnu then takes the avatar of Parashurama, Sri Ram and Sri Krishna and annihilates all those people who were practice Adharma on earth.
Parashurama wiped out the cruel kings 21 times. He made sure that none of the greedy, egoistic and lustful kings existed on the face of earth.
Sri Ram appeared to set an example of how a king should rule the earth. He also killed the demons like Ravana and others who were propagating Adharma.
Sri Krishna appeared to give lessons in Dharma and also to show the people how to lead a spiritual life living in the society. Sri Krishna also annihilated numerous kings who were resorting to violence on earth.
Thus Vishnu as Gopala is not just a caretaker of cow but also the preserver and who imparts justice on earth.