In a nutshell, Buffer is a system that will let you schedule all your tweets. You simply choose at which time you want your tweets to be sent out, add the actual tweets to your buffer and then they will be shot out when the time comes.
For example, you can schedule to have your tweets released four times a day (say, 9:00, 12:00, 15:00 and 20:00). Or two times a day, or ten times a day. It all can be arranged, and the tweets themselves can be added using the provided web interface or any of the browser extensions that are available.
The actual uses a service like this one can have, now, rank from keeping a (somehow) active social presence when you are going away on a vacation, and to something as bizarre as making others think that you never sleep. It is bizarre, but it is all allowed. Yet, bearing in mind that Buffer is a paid service I can visualize people putting it to purposeful uses most of all.
BufferApp.com In Their Own Words
Buffer makes your life easier with a new way to schedule Tweets,
Why BufferApp.com It Might Be A Killer
It is a very practical way to keep your followers interested when you know you will be unable to tweet for some time.
Some Questions About BufferApp.com
How far ahead can you plan things out using Buffer? 
