Usually, the camera’s exposure time is set by the shutter speed you’ve selected, so that the exposure ends automatically. But in Bulb mode the shutter stays open for as long as the shutter button is held down, so it’s used a lot for night photography or low light photography, where exposures can range from 30 seconds to 30 minutes (in moonlight). In the old days you’d use a cable release with a locking screw; these days you’d use a remote release with a bulb mode (using a remote release means you don’t risk moving the camera by keeping your finger on the button).
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