The Northuldrans live in a forest bounded on its Northern side by a high mountain wall. Before the dam was put into place, the river ran over a waterfall and directly through their forest, cutting it into two and diminishing the amount of land available to graze their reindeer.
By building a dam and diverting the river's course around the forest, the Northuldrans gain a significant amount of free land and no longer have to make crossings from one side of the river to the other.

Obviously the dam represents a substantial benefit to them, albeit at the cost of angering the spirits and harming the balance of nature.
The film's Official Deluxe Novelisation concurs with this.
To demonstrate their goodwill and friendship the Arandellians built a mighty dam in the enchanted forest. They placed it on the river that flowed into the ArenFjord, the deep blue body of water upon which Arandelle Castle had been built. The dam connected all the lands and made it easier for the Northuldra and their reindeer to roam.
Interestingly, the latest tie-in novel (Frozen 2: Dangerous Secrets: The Story of Iduna and Agnarr) suggests that the Northuldra weren't actually that keen on the dam, and may simply have accepted it as a gift from a pushy (and clearly more advanced) neighbour.
They’d come to us years ago with an offer of peace and goodwill, promising to build a mighty dam to help us water our reindeer and keep our land fertile and fresh. I didn’t really understand the whole thing, and I wasn’t sure our elders were completely sold on the idea at first. But in the end, they came to an agreement and the dam was built. That day we would feast together to mark this new alliance between our people and theirs.
It was a day to dance and sing and celebrate the beauty of the forest.