Twitter users have developed the habit of defying the original purpose of the app and going on multi-tweet rampages known as tweetstorms, so 'Thread Reader' helps interested users to make sense of the social media havoc. The third party system aggregates tweetstorms automatically and collects them onto a single, easy-to-read page.
One of the big problems with tweetstorms is that Twitter simply isn't designed to accommodate that style of usage. Tweets on a user's profile are organized with the most recent at the top, so users need to scroll all the way down to search out the beginning of the thread and then work backwards. Thread Reader not only does that work for users, but it also lays the entire rant out on a single page -- which is far more readable that cutting it up every 140 characters.