Exogamy

Exogamy is when a person ought to marry someone from outside a certain group.
Biological.
In most places, exogamy is practiced in order to avoid incest, which is taboo in many groups across the world. This is to avoid some of the effects that endogamy creates such as genetic diseases. If two parents have very different genes then there is a lesser chance of a child inheriting both copies of that gene.
Cultural.
Exogamy is used to make a connection between two groups. For example, when a person marries another they become part of the family. This connection may be for reasons of inter-group cohesion. In other words, creating an alliance with another group.
Dual exogamy.
Claude Levi-Strauss, an anthropologist, argued that when two families are connected by the marriage of two people it creates more relationships for trading other things such as food. It also means that they have more people to rely on. In order to keep this going, groups will trade sisters/daughters for wives and trade back in return.