If Muhammad later ban adoption anyway, what was the point of this marriage?

He did it to show that men could marry their adopted son's wives. I.e. men could bang women their adopted sons had banged. Then later adoption as a whole was banned. Then of what significance is the marriage?

So to show adoptive sons are not the same as biological sons, your prophet had to bang the wife of his own adopted son. Do you not see the problem here?

If you adopt a person you stand as a father to them does not matter if it is biological or not. Is it okay to bang your adopted daughter then? Since it is not biological?