I like my back bulkhead with a little room behind the back cockpit rim. I shove things there. (extra water bottle) etc.

As the boat width increases, (>21" or so) those back 'corners' get more and more useful. A wide mouth thermos or soft water bottles can fit there. The cockpit sole behind the seat is a good place for water bladders when touring, too.
And the sponge and paddlefloat need a place to stow, too.
Good reasons to put in padeyes or loops for lashdown points / shock cord threading when you are planning. It's easier to do before the seat is installed!
:-)
Another useful storage spot in a boat that has enough width is between the cheek plates (seat hangers) and the hull.