In the traditional content-delivery model, a content provider such as a television network would send all content downloaded by users from its servers directly to each user. Because content providers are charged for Internet access based on how much they use their Internet connections, the cost of such a service can be prohibitively high for them, especially if the content is large and is provided without charge.
P2P networks solve this problem by turning each user who downloads content into a short-term server that can forward the same content to other users–from peer to peer. In this model, a user in Boston downloading a video from a television network in New York will actually receive most of the video from peers–selected without regard to their location–in other states or even other countries.
Individual users generally pay a flat rate for their Internet access regardless of how much they use it, but their Internet service providers (ISPs) must pay more to carry additional data or to route data long distances or onto the networks of other ISPs. Thus P2P technologies shift the cost of providing content from content providers to consumer ISPs–who must cover the cost of a form of use that was not anticipated when the flat-rate Internet pricing model was developed.
— Christopher Petro