It's holding the last 30 seconds or so in memory, so then when you see something very fleeting and very cool that you'd like to take a picture of, push the capture button within 30 seconds, and a burst of frames are stored.

Perfect for when the kid or cat does something precious that's never going to be repeated, or for when you're driving along and see a bird in passing, or for sporting events, or ... well, any high-activity situation where the reflexes of the photographer are not fast enough to capture an unexpected event in real-time.