Runeard built a dam for the Northuldra tribe and presented it as a gift to them. In this he had an ulterior motive, but to achieve it the dam had to plausibly seem like something beneficial that they would want.
However the Northuldra are nomads who get most of their protein, clothing and other things they need from reindeer. They don't practice agriculture and we don't see them fishing. It's hard to see how a dam could actually be expected to serve a useful purpose to them. Yet Runeard clearly expected them to think it would. What benefit did he expect them to see in it?