Re-useable code
Re-usable code can save you time and coding effort. Using the same code over and over again can help you maintain unified functionalities and the same look and feel throughout the application for example, using the same code to validate a date in a specific format, using the same query to display a select list, or using the same unique color scheme with the application Theme are just a few possible examples.
Another advantage is that the re-useable snippets of code are usually concentrated in specific and known locations. That makes application maintenance easier.
Shared Components
The Shared Component module includes several sections that allow us to re-use specific APEX components. Whenever you need to use the same APEX object or functionality more than once, you should check if you can define it as a shared component. Notable examples are the List Of Values (LOV), Application Items, and of course, the Templates.
Shared Components can easily be accessed from the Action...