Cost-Benefit

Itunes allows users to share their media libraries over a local network with other Itunes users. This sharing feature provides users with the ability to play the music and video files from another without any sort of cable connection.

It is important to note that the sharing feature is disabled by default, presumably for security reasons. As shown in the screenshot below, users must navigate through the Itunes menu if they wish to enable sharing.

This is a perfect example of the cost-benefit principle. The Itunes designers knew that those wishing to share their libraries would perceive the benefit of a having a shared library as outweighing the cost of navigating through a series of menus.