Does your inbox ever fill up with emails that you know you won't respond to for at least another week? If you archive them, you'll just forget to ever write them. Even if you stick them in a special "open me on ..." folder, you're unlikely to attend to them.
Do you ever want to delay sending a message by a few minutes? A few hours? A few weeks?
What if you send an email and want to make sure you're reminded about it if no one responds?
If you have ever had any of the problems above or are just plain curious, you should try Boomerang for Gmail. It's an extension available for Firefox or Chrome and I highly recommend it. You can read their FAQ here or download it here.