Separation of Concerns and abstraction library.
UNITY, MONO, Alternate GUI (GTK, Etc)
XYZ allows for creating a Model/Glue/View application where all functionality aside from the direct GUI code is handled in abstracted layers.
XYZ offers wrappers to build system functionality on to integrate with business and view logic. These wrappers allow for replacing backend source such as Libraries, Frameworks, etc or Reimplementation of such code without changing the consumer code. In otherwords, it localizes and abstracts "heavy lifting".