When you focus on Done, your daily work has focus: get stuff done! Your daily scrum has focus: point to tasks on the board that you got Done yesterday and that you will get Done today. What is preventing us from getting tasks Done? A ha! Now we have focus, the ability to identify impediments, and the ability to remove them, every day.
When you focus on Done, your sprint has focus: get your sprint commitment done. Your sprint retrospective has focus: either we got the sprint Done, or we didn't. If we got it Done, what did we do that made us successful? Let's repeat those things in the next sprint. If we failed, why? What impediments prevented us from getting done? A ha! Now we have focus, the ability to identify impediments, and the ability to remove them, every sprint.
Done is the crux of Agile because Done focuses you on getting done, and on removing impediments and making improvements. Removing impediments and making improvements is the crux of improving your velocity and becoming a high performance team. The ability to get stuff Done is what Agile is all about.
If you don't know what Done means, you can't get anything done. What is your definition of Done? Is it the same as your customers' and managers' definition of Done? How does your definition of Done help you get stuff done, and how does it help you remove impediments and increase your team's velocity?