created by and modified by for a item

when we create a list item we can change the created by and modified by like this :

item["Author"] = userValue ; //InternalName of "Created By" field is "Author"

item["Editor"]=userValue ;  //InternalName of "Modified By" field is "Editor"

but we we upload a file to document library, we will get two columns Document Created by and Document Modified by.

the type of these two columns are text .

my question is what these two columns do ?