Enabling or keeping productivity going is also important. That is what people watching line and fixing minor things are paid for. Alternative is that someone would be pinged from home and they would drive to line each time. While during time productivity of line was lost.

Labour productivity in economics is output per labour hour, it doesn't matter what is doing the work. As an extreme example, if robots took over and human labour is no longer required for anything then (human) labour productivity would be infinite, it doesn't matter if no humans are actually doing any labour.