Anonymity is a concept that has existed throughout history. We all know of the anonymous letter, anonymous donations to charity and the anonymous tips to police.
In every case, there is no connection between the message and an identifiable individual. The message stands alone.
The advantage of anonymity is that the sender is without responsibility, or rather, they are only responsible for themselves. This can be beneficial if one does not desire the attention that a controversial point of view, large donation, or revelation of important information would bring.
Anonymity gives security, freedom and a lack of responsibility. Complete anonymity is difficult, indeed almost impossible, to maintain in the real world - at least if we are to interact with our surroundings.