Alma 3:8

and this was done, that their seed might be distinguished from the seed of their brethren, that thereby the Lord God might preserve his people, that they might not mix and believe in incorrect traditions, which would prove their destruction.

This was done so their children would look different from their brothers’ children. This way the Lord God could keep his people safe. He did not want them to mix with people who believed wrong things that would destroy them.

He did this so their descendants would be easy to recognize—to keep his people from mixing with them and adopting false beliefs that would destroy them.