What is the purpose of the President's cabinet?

Question

http://usgovinfo.about.com/blcababout.htm

Read especially the section entitled Does the Constitution provide for the Cabinet?

Also these:
http://bensguide.gpo.gov/6-8/government/national/cabinet.html

http://usgovinfo.about.com/od/thepresidentandcabinet/

http://en.wikipedia.org/wiki/United_States_Cabinet

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The purpose of the cabinet is to advise the president on matters relating to the duties of their respective offices.