More than likely as others have said it is lunch. If the job runs 3 shifts then probably at one point they had a 20 min paid lunch. By going to a 30 min unpaid they only loose 10 min productivity but save 20 min pay each day. If they still had 8 hr work days with the 30 min unpaid lunch then they would end up with 2 people at most stations at shift changes for 30 min.
3 shifts at 8 hours = 24 hours, add a half our lunch to all three shifts, that equals 24 hours on the clock, but 25.5 hours at the building. Simply remove 1/2 hour from each shift and you have seamless shift change with no overlap.
Now this would only work on a non automated, continuous flow, production line. With an automated production line some overlap is needed.