A green card gives assurance that the holder can return to country of origin to pick up residence if he so desires.A green card gives legal residence in the States without having to renege on his original citizenship thereby having the chance to return to his country of origin. He is getting the best of both worlds. Once an immigrant accepts US citizenship he is no longer welcome to return to his country of origin.
Well, since I've been giving a hard time to the open border folks, I'll take a break to say that that isn't even close to an accurate statement.