In the Chester, Saint John photo (4th one) you can clearly see a non-grouted horizontal line across the stone walls. I’ve seen this in many cathedrals and was w

In the Chester, Saint John photo (4th one) you can clearly see a non-grouted horizontal line across the stone walls. I’ve seen this in many cathedrals and was wondering 1) what it’s for and 2) if that’s part of the original construction. My guess is that it’s some sort of contemporary mitigation to avoid cracks?