Now I'm not quite suer I have it right, because the spinning classes I go to are in French... but "courir" means to get up out of the saddle and pedal at a VERY high cadence.

The value? In terms of muscles, you tend to use your hamstrings and calves more when you stand on the pedals than when you sit in the saddle. Cyclists' hamstrings tend to be under-developed, and that can be a disadvantage in out-of-saddle climbing and sprinting. The high cadence helps train your muscles for leg speed, something that cyclists always have to train for or lose [or, so says Greg Lemond]. The combination is supposed to give you leg speed and strength.

In terms of CV conditionning, you'll note that your heart rate usually goes way up when you get out of the saddle. If your spinning class is doing threshold training, that can be a good way to quickly get you up to your anaerobic threshold. At my spinning class yesterday, the instructor had me up to 175 for five minutes. It was not fun. [I think my maxHR is around 190].