Butterfat contains arachidonic acid. As detailed in my PUFA Report, arachidonic acid is critical to fertility in both men and women. In men, it is necessary for the production of testosterone and sperm. In women, it is necessary for ovulation, and when it is time for birth it is necessary for the induction of labor.
The best sources of arachidonic acid are liver and egg yolks. Butter is a decent source, and other animal fats also have small amounts.