When the files in your project have a lot of content in common, you often need to resort to duplicating this content across all of the files. For web site authors this can be e.g a common page lay-out or menu, but other candidates are e.g. standard headers and footers you add on to files you send out the door.
When you later find out you need to update the common part, you find yourself with a lot of work on your hands. And if you forget to perform the update in one particular file, or if you make an error while performing the update in a certain file, your project will be broken.
This situation calls for common TExpressive file templates - define the common content in one location (the template file), and re-use it in all your project files. This way you only have to modify the template file when updating the common part, and all is done without having to worry about some files missing the update or getting updated incorrectly.