For a start Bert you hardly know nothing. Plus you've had the sense to ask the question.
Perches are set higher as birds roost instinctively at the highest place, generally (not our Orpingtons). Nest box roosting breeds mite and lice. When these get established you will know because the hens gradually won't lay in them. Then the eggs are left all over the place and the next stage is egg eating, which in the worst case could result in the destruction of your flock because it is very difficult to stop.
Put curtains in front of your nest boxes to discourage egg eating and roosting and to encourage them to always lay there. We make them from Wikos car mats slit 15mm wide to 2" from the end. All our nest boxes have them fitted and we never get roosing in the boxes, egg eating or laying anywhere else.
Black Rocks are great. Our little Annie was so friendly, used to talk to me every day when I passed. Very bright as well (except for Daphne).