Explanation/Reference:
Round-trip engineering is the ability to effectively perform both forward and reverse engineering to seamlessly transform the lifecycle assets. Forward engineering is very useful the first time that code is generated from a model. It saves much of the mundane work of keying in classes, attributes, and methods.
Reverse engineering is very useful both to transform code into a model when no model previously existed, as well as to resynchronize a model with the code at the end of a change.
Note:
During an iterative development cycle, once a model has been updated as part of an iteration, another round of forward engineering should allow code to be refreshed with any new classes, methods or attributes that have been added to the model. Source code generally contains much more than the model and tools must be very adept at reconstructing the source code that existed prior to the new round of forward engineering. At minimum, the modeling tool should successfully support forward engineering the first time and reverse engineering throughout the process.
Reference: Oracle Reference Architecture, Software Engineering, Release 3.0