The shaft can not come out while driving, because it is held in place by
the wheel hubs, which are in turn held in by the wishbones mounted on to
the subframes. The rubber boot essentially helps keeps the grease in,
and the dirt out, and thus prevents the CV joints from prematurely
wearing out due to contamination. This is my understanding, and I am a
layman. Experts on the group please let me know if I am correct