What are the Call Back Events in UI5/Fiori?

A callback event automatically invoked by a framework when an operation or activity completes. callbacks allows users to provide custom code to be executed by the Framework.

Call back events are basically asynchronous mode of processing.. They are executed only when the application life cycle is over... Similar to that of Update function module concept..it is basically nothing more than asynchronous mode of processing... It is useful because OData uses aajax call and does not load data immediately, request are only bundled to be executed later.

Eventually odata calls are made only after your application lifecycle is over. So at that point to manipulate the value you need a function that triggeres after the OData model execution is completed