Just going off the pictures and description it looks like a sleeping bag pocket that I had on a couple Lowe Alpine backpacks.- Sleeping bag compartment - typical to compress your bag in a compression style stuff sack - is the thought to forgoe those and build it into the bag, or?
There was an internal zipper that would close off the bottom section of the bag and I would stuff the sleeping bag into the compartment and not use a stuff sack. I can’t really tell you what the advantages were over using a stuff sack outside of it helped keep things organized in my pack.