The most important thing the pork system does is to generate revenue. That would not be federal revenue; it would be political revenue. The system is simplicity itself: the senator legislates the earmark; the beneficiary generates the revenue - for the politician.
There are certain rules that regulate the money stream. First and foremost, the gift must be to the politician's campaign, not to his or her person. Second, neither donor nor recipient can articulate in any way any link whatsoever between the legislative act and the donation. In this wonderfully simple system, the petitioner makes known his wants, and then the legislator legislates. Before, during, or after the legislative deed and without either party visibly offering or soliciting a gift, the campaign donation mysteriously appears. If it doesn't, the next time around, the legislator may be "too busy" to help the petitioner. If the legislator doesn't produce, the expression of appreciation just might "fall between the cracks."
The published rules are carefully written to make this fundamentally corrupt system perfectly legal. There is hardly a single sitting member of Congress who has not repeatedly benefited from it.
http://www.truthout.org/article/is-king-pork-dead