set_offset() is another way to move the camera relatively from its position, without actually modifying the transform of the Camera node. It should be allright to call it on every frame.
It is intented to be used as a secondary positionning parameter. Screen shake is given as an example because it typically doesn't changes the camera's looking target, but has to make it move a bit around it. You could do everything with
set_pos(), but using
set_offset() can make logic work independently from the transform.
I think this is also useful if you enabled smoothing, because
set_offset() won't be affected by it, unlike
set_pos() (didn't tested yet, thought).