Alternative boot uses
By pressing the user button prior to power on, an external microSD card can be used to reflash a BBB to the factory state. This is useful if the flash is corrupted, if a used board is purchased, or if you want to reset a board between class sessions. Images to reflash a BBB can be downloaded at www.beagleboard.org.
More advanced uses of the alternative boot option include rapid development by allowing a desktop or laptop to preconfigure a microSD without having to wait for the image to be written to the onboard flash memory.