identity mapped paging

I am not clear about the use of identity mapped paging while paging is being enabled by the operating system. Also I don't understand at what point are the identity mappings no longer useful.According to this article http://geezer.osdevbrasil.net/osd/mem/index.htm#identity - "The page table entries used to identity-map kernel memory can be deleted once paging and virtual addresses are enabled." Can somebody please explain? Thanks Vaibhav Jain