The two most likely answers to this are that the managers should be A) walking around the production areas and B) talking to workers.
Management can be said to be partly an art and partly a science. The use of computers and smart phones and such is typically associated more with the science aspect of the job. When managers are using their electronic devices, they are probably doing things like crunching numbers or trying to find greater efficiencies. These are very important things to do. A manager must try to make the physical and measurable aspects of the job work as well as possible.
However, this is not all there is to management. Management is also about people and about paying attention to what is actually going on “on the ground.” By walking around the production area, the manager might notice things that could be improved or that are starting to go wrong. By talking to workers, the manager is keeping in touch with the thinking of the people who are actually working. The manager is also helping to keep morale up by showing the workers that he or she is paying attention to what they are doing.
Thus, managers need to stay in touch with the people and the workspace that they are responsible for. They cannot simply work on their electronic devices all the time.