I'm curious as to why it's more efficient to bring the shelf to the picker than take the picker to the shelf.
Those robots could just as easily be ferrying around the pickers.
They could but that would make the process into a serial process. Why waste the time bringing the picker back and forth from the shelves to the belt? If you have enough or fast enough robots, it is more efficient to have them timed so that another shelf arrives just in time for the previous shelf to be removed.