It is not easy to follow these modernization trends for big applications (like, for example, home banking services), with hundreds or even thousands of user interface elements. Using a GET approach allows you to automate the adaptation of UI elements to new technologies and frameworks without the interruption of the ongoing functional development. By integrating it with the development lifecycle, the number of development recycles can be reduced.
Just imagine that you have to apply a specific CSS class to some specific HTM fragments. Doing it manually means a developer has to open all the files. Even if it is a simple operation, the time and the risk of manual modifications can become very high with respect to an automated solution.
With respect to this simple case, much more complex modernization operations can be realized. These include technology migrations from server-side HTML generation frameworks (like JSF and Co.) to new client-sides such as AngularJS, Bootstrap or React.