Interacting with your readers is not only useful in terms of building a dedicated readership, it can also enable you to get feedback in an instant and very frank way. People are often too busy or simply too lazy to leave a comment on a blog, or a review after they've purchased a book. But a quick tweet or a sentence via Facebook is easier and, somehow, less formal.
Moreover, followers and fans who regularly read your work, whether it's novels or articles, begin to feel like they know you. So, interaction via social media becomes more like chatting to a group of friends.
Of course, this informality and instant communication is not to everybody's tastes, and as mentioned above, social media is not a necessary component of a writer's success. Additionally, because a consistent presence is required, there really is no point in delving into these things unless you're likely to enjoy them. A halfhearted attempt at social media, which I have been guilty of on more than one occasion, simply doesn't work.
However, for users of Twitter, Facebook and other social media platforms, the intimacy and immediacy of communication is a great part of the attraction. And, for some writers, good social media proves a hugely beneficial tool in marketing their work.