Why is knowledge about human biological evolution important to social sciences like cultural anthropology?
As humans evolved, we were able to develop culture. Culture is what separates us from the animals. For example, animals can make sounds and even use these sounds to communicate but humans have the functionality to produce words and complex sounds. That functionality, of the mouth, throat, tongue and vocal cords, was evolved over time. Since language led to culture, the evolution of the ability to speak and the evolution of the actual sounds, meanings, syntax and grammatical structure are intertwined.
One of the goals of social sciences such as cultural anthropology is to understand why societies have the customs that they do. For example, one might ask why it is that human cultures often tend to value monogamy in marriage (some don't of course, but many do). In order to understand such issues, it is important to at least consider the impact of evolutionary pressures on human beings. It might be that our inclinations towards (or away from) things like monogamy are connected to the pressures that made us evolve.
Looking at monogamy, for example, you might wonder if monogamy is somehow part of what we have received through evolution from our ancestors. What if, for example, it was biologically beneficial to be monogamous -- if males who were faithful were more likely to be able to mate than those who were not. Or if females who were faithful led their mates to do a better job of providing them and their children with food.
Because of this sort of possibility, it is important for social scientists to at least think about evolution when they try to understand why our cultures are the way they are (and when they think about trying to change our cultures).