Simple really - productivity. On every shift you lose time starting up and getting everyone to work. This happens twice a shift (start of shift, coming back after lunch). There is a gap in time from when everyone clocks in to when they actually start producing. This time is referred to as "stand-up" and is when the manager covers safety, quality, admin messages, announcements, etc and also performs stretches. Multiply the time spent doing this by the number of minutes your shift is producing absolutely zero and you start each shift in the hole. You also lose some towards the end of a shift (everyone stops just a bit early until you start dropping the hammer). Knowing this, the fewer the shifts, the less "lost" productivity.