Last night it occurred to me that perhaps people do it as a warm-up for the real thing because I saw one dummy ride with loads of people on it in the Live tab. Then when I did the bkool videos advanced search I joined the actual ride with about 6 minutes warmup left but I was the first, with Rick joining shortly afterwards. Then nearer the end of the warm-up there were loads joining so I thought they were probably jumping off the dummy ride onto the main one having had a decent warmup and a good chance to figure out their strategy.
I'm probably wrong with all that though.