Well I guess as I am a "Would-Be Writer", I can only comment from a readers perspective, but I do follow a number of writers on twitter, the author of this blog amongst them.  Whilst some of those authors I follow because I have read their books, and want to keep up with what they're doing, how their next book is coming along etc. I also follow writers who's books I've never read.  I am however more likely to buy one of their books if I think they are a human behind their tweets.  For example it is actually them tweeting and not someone paid to do it on their behalf, and they are tweeting about not just what they are writing about, and that they interact with their followers (David is a very good example of an author who has this spot on, IMO).  I have bought books as a result of this approach, and discovered many wonderful authors as a result - this tends then to lead to following other authors etc etc.

One final point from the "would-be" perspetive, I also follow for hints and tips.  Not because I want to push my "as yet unpublished masterpiece", but because one day I do want to get published, and any hints/tips/advice I can find along the way from those who are already having some sucess is great.  There is however a major caveat to this final point, never ever push, apart from being downright rude there is (or at least it seems there is to me), an etiquette to follow, and that means treat people as you would like to be treated yourself.