I often wondered where the other kids came from but never really cared or thought much about it. I figured not everything was written in it. I thought I read but could be wrong as it was a long time ago that Adam and Eve had other children after Seth and they were girls. Maybe Seth committed incest, who knows? In the grand scheme of things, it doesn't really matter.
I think it matters because it points out the hypocrisy of organized religion, especially Christianity