They let you create class members of sorts. If you think of your widget as a class that is. You can define any kind of data you want and modify it in the orchestra using cabbageGet/cabbageSet. The possibilities are vast. One recent use for them popped up in this discussion.
It uses the cabbageGetWidgetChannel opcode to grab a set of widgets from the GUI. There was no way to do this in the past, it was all or nothing.