Basically having the ability to have other 3D section boxes inside the initial box.
As Marvi suggested in a comment in another post, if there is a toggle button to invert each of the boxes functions to cut everything that is inside them (like void extrusions), then this can become a very powerful tool in visualisation.
It also helps with modelling big sites that have complex buildings with lots of objects inside them, you can just hide everything inside the building with a "void" box.
https://forums.autodesk.com/t5/revit-ideas/section-box-not-just-box/idc-p/7413410/highlight/true#M14...