(B) In old-fashioned flash-bulb photography, the bulb must be ignited a few milliseconds before the shutter is opened, to give it a chance to get going.
Alternatively, the shutter must be opened for a relatively long time, to make sure that the bulb gives out its light while the shutter is open.
Nowadays, the camera 'B' position is used for time exposures, it being a setting where the shutter will remain open for as long as the shutter button is held pressed.