The pin assignment (pinout) describes which signal may be obtained from some position (mostly pin) in the connector. Knowing pinout is absolutely necessary when you are trying to inter-connect different hardware, such as computer and cell phone, computer and other computer, PDA, GPS, IC's etc.,
It's also useful for diagnostic of hardware problems, making self-made interface cards, obtaining signals and voltages from some device and many other hadware-related things and tweaks.