Why did we get this detail?

I have long assumed that we got such a detailed account of the last generation of the neighbors leading up to their destruction simply because Mormon didn’t advertise his own original record. Today I am thinking that there may be more reason than mere expediency. I wonder if we need to see how the people persisted in wickedness even as they had repeated opportunities to repent and even as they years at peace asking their decades of wickedness and refusing to repent.