On each of the boards I tried to use the space to provide additional features to complement the platform they were designed to fit.  For most platforms, this meant including a microSD connector, but I went above and beyond for the Itsy Qwiicky.  Not only did I include a microSD connector, but I was also able to include a battery connector and charger.  All of this and with castellated pins that fit neatly as a little backpack behind the Itsy Bitsy with or without the header pins installed.