Why does photography need a backup machine?

Because the SLR is heavy, there is a spare machine that can be easily carried, so it is not a bad subject. In addition, once there is a problem with the host, the standby machine can also stand for a while.

In fact, when you play photography, you will take two cameras out ... it's too much trouble to change the lens of one camera. From this point of view, it doesn't matter whether there is a backup machine or not.