Marriage between royal family was often a way to secure / strengthen an alliance between two monarchs. For example, according to Wikipedia
Marriage between dynasties could serve to initiate, reinforce or guarantee peace between nations. Alternatively, kinship by marriage could secure an alliance between two dynasties which sought to reduce the sense of threat from or to initiate aggression against the realm of a third dynasty
I wonder, how this would work from the point of view of the dynasty that supplied the princess? For example, if you marry off your daughter, and she lives in the other monarch's family, of course it will somewhat restrain you from attacking them. But how does this guarantee (or at least increase the confidence) that the other monarch will not betray you? If anything it seemed the daughter can be used as hostage, a bargaining chip, or even mistreated by the other monarch in future conflicts.