The killer use-case for blueprints is to organize our application into distinct components. For a Twitter-like microblog, we might have a blueprint for the website pages, e.g. index.html and about.html. Then we could have another for the logged-in dashboard where we show all of the latest posts and yet another for our administrator's panel. Each distinct area of the site can be separated into distinct areas of the code as well. This lets us structure our app as several smaller "apps" that each do one thing.
Note Read more about the benefits of using blueprints in "Why Blueprints" from the Flask docs.