chrisg-optical wrote:
Not only for redundancy in case something goes wrong with one, but also to avoid having to constantly change lenses, so the camera is ready to shoot immediately.
Back in the film days, multiple bodies were also used for different films (B/W & color, ISO low and high, slide vs print, etc.), and also to avoid having to change rolls during active shooting.
Pro photographers certainly get a workout with all that gear!
Even I, a genuine amateur, kept my backup camera loaded with B/W; if my primary failed, I always could have wound the B/W film back into its can, but in the meantime I had it available if I did want to use B/W.