Why were the Ancient Romans always exchanging their daughters for marriage?

When you read about the ancient Romans especially during the late Republic and early principate, you will see that they were always exchanging daughters with each other. Sulla gave Pompey his daughter for marriage. Then they divorced Then, Pompey took Caesar's daughter for a wife. Caesar took Sulla's... show more When you read about the ancient Romans especially during the late Republic and early principate, you will see that they were always exchanging daughters with each other.

Sulla gave Pompey his daughter for marriage. Then they divorced
Then, Pompey took Caesar's daughter for a wife.
Caesar took Sulla's grand-daughter for a wife.
Caesar earlier married the daughter of Cinna, Marius's co-counsel.

Earlier we saw this sort of running around with the Scipio and Gracchi family.
Later you see it with the Julio-Claudians.

Why is this? Why were they always handing out their daughters to each other?
Certainly not all families and people were like this.
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