These were not work donkeys or pack animals. These were donkeys on which one rode. And riding on donkeys rather than walking was a sign of wealth and status in those days. Monarchs often rode on donkeys. If Jair lived in modern times, it would read, Jair had thirty sons, and each son drove a BMW.
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4 And he had thirty sons who rode on thirty donkeys, and they had thirty cities, called Havvoth-jair to this day, which are in the land of Gilead.