I would suspect that there is an empty space there.These have their own roof and window .
You can see them in the upper left - sorry for the lousy picts! I should have taken a pict deeper from the inside.
View attachment 166105
Poke a hole and the side and see what you have?
Having owned several older houses with steep roof pitches; I have found lots of little areas that the framers made 90 degree angles and small knee walls for no apparent purpose.
I always assumed it was to make it easy to drywall those areas.