There are two main reasons for first-cousin marriage:
1) Bride-price (mahr): the groom is supposed to pay a dowry to the woman, which serves as her alimony in case of divorce. Marry a first cousin, and the money stays in the family.
2) Avoiding dividing clan loyalty: marrying somebody from the same clan ensures that the loyalty of the children is not divided.
Not that it's not just first-cousin marriage, you are supposed to marry your first cousin on your FATHERs side, which concentrates male-oriented recessive genes even further.
Even Abraham commanded that the daughters of Israel were to marry withing their tribe that the inheritance “be not removed”