The most important detail that the narrator gives about Ibzan is that he makes marriage arrangements for all sixty of his children. And all sixty spouses were from outside his tribe: he gave his thirty daughters to outsiders, and thirty outsiders he brought in for his sons.
Why did he do it? In a tribal social context, this is a way to widen your connections and influence. These marriages cement clan alliances and extend the scope of Ibzan’s political influence.
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9 He had thirty sons, and thirty daughters he gave in marriage outside his clan, and thirty daughters he brought in from outside for his sons. And he judged Israel seven years.