Krsna says among cows I am the surabhi. Kama-dhenu is also known as surabhi. Surabhi cows inhabit the spiritual world, and, as stated in Brahma-samhita, Lord Krsna is engaged in tending these cows: surabhir abhipalayantam [Bs. 5.29]. One can milk a surabhi cow as often as one likes, and the cow will deliver as much milk as one requires. Milk, of course, is necessary for the production of so many milk products, especially clarified butter, which is required for the performance of great sacrifices. Unless we are prepared to perform the prescribed sacrifices, our supply of the necessities of life will be checked. Bhagavad-gita confirms that Lord Brahma created human society along with yajna, the performance of sacrifice.
After Lord Siva drank all the poison, both the demigods and demons took courage and resumed their activities of churning. Because of this churning, first a surabhi cow was produced. Great saintly persons accepted this cow to derive clarified butter from its milk and offer this clarified butter in oblations for great sacrifices.
SB 8.8 Summary