The crown board/inner cover has a hole in it for a feeder etc. If you don’t use a hive mat, bees can go through the hole up into the roof. I always cover the hole in my hives, but some people like to leave it open as an early indicator of overcrowding if the bees start building in the roof. My bees are always challenging, and they just like to build up there anyway, given half a chance, so I block it off.