When does immersion require standing?

Walk the floor of any VR conference and it is rare to see someone sitting down while immersed. Certainly fitness, sports and physical training apps often require full body movement, but what about travel, meetings, games, and other experiences? Is it because body direction needs to be used as an input? Is it to mitigate cybersickness? How do you keep users safe while standing? Can multiple students in a classroom avoid bumping into each other? Are there times when experiencing immersion while seated would be desirable? Safer? What will it take to enable a moving experience while seated?