It's also an acknowledgement that the anglo-saxon way of law is not the only way.
[1] See, e.g., Lawrence B. Solum, Legal Personhood for Artificial Intelligences, 70 North Carolina Law Review 1231, 1239 (1992) (https://scholarship.law.unc.edu/cgi/viewcontent.cgi?article=...)
The law was recommended by the Waitangi tribunal which is a body that redresses historical grievances by Iwi.
Not only is the river now it's own entity which is not owned by anyone local Iwi received $80 million from the government and a $30 million dollar fund to improve the health of the river.