Commands are a very important concept in the GEF MVC framework. To customize your GMF application it is very important to understand what they do and how they integrate into the overall framework. When you integrate in the command framework the framework provides hooks to add plenty of functionality, for example you can add undo, redo and transactional operations by implementing the corresponding interfaces. Thus you can focus on the core of the application logic and forget about boilerplate code.
Image credit: Jason Devaun