Let us start by discussing sharing in general. Sharing is simply one person giving something they have to another. With digital materials, that gift is an exact copy. When one person desires something, they go to a person that has it, and asks for it. If they have that item, in this case bandwidth, they file-share.
Especially in cases where there is perceived value in collecting, some people will have lots to share and will find themselves surrounded by eager people. This can cause problems when the collector cannot keep up with demand. Decentralization is one means to alleviate this problem, especially in cases where it is possible to ensure that multiple copies of a popular item are available from multiple sources (even simultaneously, as with multi-source downloading).
Concepts like hoarding come about where the one centralized person will collect and will not later freely give away what was given. Barter and ratio systems evolve in order to reduce the impact of hoarding. Under these systems, a person will only share when they can expect to get something in return.
The concept of being tagged and tracked is one which has been a reality for some time. Personally identifiable information is legally associated with a person's actions in order to verify their identity.
Think credit cards: these have to be associated with the purchaser, otherwise the credit card couldn't send a bill for those purchases. This concept has also entered into the computer world, and many people are fearful of advertisers tracking their virtual "movements". While advertisers claim that this is to target more applicable or interesting ads, many people argue that they never want to see another ad again, and certainly don't want to be tracked without explicit permission. This desire for anonymity has spilled into file sharing such that some clients have encryption and obfuscation functionality to protect their users.
Moving closer to today, there has been a great discussion over perceived and actual legal issues surrounding file sharing. In circumstances where trading partners are in different countries with different legal codes, there are significant problems to contend with. What if I, in Canada, wish to share a piece of source code which, if compiled, has encryption capabilities? In some countries, a citizen may not request or receive such information without special permission. In these cases, how can I be expected to ban certain countries or persons? Can I even identify these people? Is a warning label good enough? There have been cases where information shared publically (due to local law) has had the author jailed in a foreign country for breaking foreign laws! This is a complex topic indeed, and has been justification for an additional push in [anonymity]] functionality.
Decentralization has been widely pushed over the fear of having a centralized network attacked, either by legal disputes or hostile users. One argues that a decentralized network has no body to attack, only individual ethereal members. While the foundation protocol of the Internet TCP/IP was designed to be robust and withstand concerted attack, file-sharing and peer-to-peer systems are proving even stronger.
So today we are left with a slew of clients with functionality designed around making sharing files more effective, both in the real sense of uploading and downloading (like anti-hoarding functions) and in the more ethereal sense of being bulletproof towards legal issues (as with anonymity and decentralization).
... Continue to File sharing/Getting to the point.