The question is a familiar yet uncomfortable one. It carries the suggestion that friendship has some ulterior motive, some utility.
"Families exist because their members accept that a degree of selflessness is necessary to sustain them, and to ensure the survival of the next generation. There is no similar drive behind friendship ." [Guardian, January 2005]
"If Antony Brydon had used a social-networking application in 2000 to sell EMusic, he might have made an extra $30 million on the deal." [Time Magazine, September 2004]
As social animals (most of us) there are strong forces driving us into social relationships. Some of these relationships may be described as dysfunctional; and this implies some norm of functional friendship that some relationships fail to satisfy.
I think the purpose of friendship is a good question to explore on the POSIWID blog.
Update. Squidoo has disappeared. Archive page is still available. PurposeOfFriendship@Squidoo via WayBack Machine.
Updated 17 Feb 2016