I think the donut hole would be used as unpressurised cargo space for luggage, moving the floor of the cabin closer to the bottom of the pressurised envelope, and reducing the strength and fastening requirements of having a pressure bearing floor separating the pressurised cabin space in the top half of a cylindrical aircraft fuselage from an unpressurised cargo space below.

I don't think any aircraft are built that way though, so I'm a little confused.