If a tweet was picked up and retweeted by @problogger, for example, this would reach out to an additional 43,000+ followers, many of which are likely to retweet this themselves and hopefully create a snowball effect of RT messages across Twitter. It may also reach additional highly-followed users, such as Stephen Fry (as shown in the image above), helping to spread your message further. There will almost certainly be an overlap in users receiving tweets/retweets, but this also increases the chances of these users seeing your message as it could have been easily missed first time around.