It's the equivalent of downshift in a regular manual or automatic transmission equipped car. You could use it down the entire mountain but it's preferred that you gain some of that momentum via regenerative braking and use the brake pedal to slow the car now. After the battery reaches full charge, you can continue to ride the brake pads or use B to assist in slowing the vehicle so that the brake pads don't overheat.
Adding to Tideland's comments, some people also like the sense of control in B mode. On a long mountain downhill you often find yourself switching back and forth from gas to brakes. With B mode you can often stay on the gas, easing up to go slower, pressing down to go faster. It makes for less of a dance. Tom
On a long descent, your friction brakes could overheat, losing effectiveness. This would be bad. By using B mode from the summit, you use less friction brakes and more 'engine as an air pump' brakes. This can keep your friction brakes cooler, and you out of the runaway truck ramps. http://www.oregon.gov/ODOT/MCT/docs/EmigrantHill.pdf?ga=t As I read this, 59% of the accidents on Emigrant Hill result from brake issues. I have watched a truck use an escape ramp, it moves a LOT of gravel!
Brakes are for changing speed, engine braking by gearing down, or B mode in the case of a Prius, are for maintaining speed. The heat generated by the brakes on a long steep descent isn't good for the brake system and in some extremes cases can cause brake failure.
