Charles Ellicott Commentary Numbers 36:8-9

"And every daughter, that possesseth an inheritance in any tribe of the children of Israel, shall be wife unto one of the family of the tribe of her father, that the children of Israel may possess every man the inheritance of his fathers. So shall no inheritance remove from one tribe to another tribe; for the tribes of the children of Israel shall cleave every one to his own inheritance." — Numbers 36:8-9 (ASV)

And every daughter, that possesses an inheritance ... — The particular direction which was given in the case of the daughters of Zelophehad is extended in these verses into a general and permanent law that no heiress in Israel should marry out of her father’s tribe, in order that the inheritance might not be transferred from one tribe to another, and thus, in the course of time, the division of the land among the tribes, which was made under Divine direction, be materially changed.