Created by and modified by for a item

Question

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 ?

Answer 1

You can add refiners to your search
http://blogs.technet.com/b/tothesharepoint/archive/2013/11/11/how-to-add-refiners-to-your-search-results-page-for-sharepoint-2013.aspx
http://technet.microsoft.com/en-us/library/jj219669%28v=office.15%29.aspx
Weith of ranking model to help on which page  the document should be viewed 
http://msdn.microsoft.com/en-us/library/office/dn169052%28v=office.15%29.aspx
<BM25Main name="ContentRank" k1="1">
<Layer1Weights>
<Weight>0.26236235707678</Weight>
  </Layer1Weights>
<Properties>
<Property name="body" w="0.019391078235467" b="0.44402228898786156" propertyName="body" />
    <Property name="Title" w="0.36096989709360422" b="0.38179554361297785" propertyName="Title" />
    <Property name="Author" w="0.15808522836934547" b="0.13896219383271818" propertyName="Author" />
    <Property name="Filename" w="0.15115036355698144" b="0.96245017871125826" propertyName="Filename" />
    <Property name="QLogClickedText" w="0.3092664171701901" b="0.056446823262849853" propertyName="QLogClickedText" />
    <Property name="AnchorText" w="0.021768362296187508" b="0.74173561196103566" propertyName="AnchorText" />
    <Property name="SocialTag" w="0.10217215754116529" b="0.55968554315932328" propertyName="SocialTag" />
  </Properties>
</BM25Main>http://technet.microsoft.com/en-us/library/dn169065%28v=office.15%29.aspxhttp://technet.microsoft.com/en-us/library/jj219669%28v=office.15%29.aspx
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