I like NMA World Edition's summary...
The simple answer for why these full body scanners were installed and are being used is for corporate profit. The federal government has already spent 360 million dollars worth of tax payer money to private contractors to build all these machines. That is just the beginning.
Former Secretary of Homeland Security Michael Chertoff has been the main person pushing for these full body scanners, but he also the cofounder of the Chertoff Group, a security and risk management group. They consult with the manufactures of these ridiculously expensive machines. He directly profits from every one of these machines that are built. He is abusing his relationship with the public for personal profit. It is upsetting that nobody in the mainstream media will attack him for his obvious conflict of interest.
The reason why the TSA rules and regulations for pat downs now include TSA agents having to repeatedly touch your junk is because they want you to be uncomfortable. If it were a simple pat down, then nobody would choose the full body scanners. Everyone would ask for the simple pat down and the machines would not be used. Changing the TSA pat down procedures to include repeatedly touching breasts and genitalia is for the purpose of making people feel like the full body scanning machines are less invasive. They want you to use these machines every time to justify having them.