Why is the rails_12factor gem necessary on Heroku?

Question

I noticed that the new version of Michael Hartl\'s Ruby on Rails Tutorial says to include the following to serving static assets on Heroku

gem \'rails_12facto

Answer 1

The best answer I could find is by looking at the rails_12factor README.

There's also a discussion on why Heroku decided to include the gem: Why does this gem exist? Especially read the answer from schneems about halfway down the page.