In the book "Hierarchy of Contagiousness" the author says that one of the things most likely to make a piece of content likely to be shared is by asking for it to be shared.
In other words, a tweet that ends with "please retweet" is much more likely to be retweeeted. Likewise with people asking for something to be shared.
The simple act of asking for a share makes it much more likely that a share will take place.
Given that getting your content shared can mean more traffic from more places AND it can mean better search rankings ... when you create something really good, why not ask for it to be shared with others?