Anyone who sends out Gmail messages containing the same body text over and over again knows how taxing that can be. Fortunately, there's a great feature in Labs called Canned Responses to help combat this issue by offering the ability to have pre-built responses directly within the compose window.
Let's say, for example, you're out of town and won't be able to respond to most email in a timely manner, but don't want to set an away notice so everyone gets the same response. In that case, you could have several different canned responses ready to go for different purposes. If a client or partner emails, then you could have an appropriate, professional message ready to go.