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

In the Getting Started with Rails 4.x on Heroku, we can read this:

Heroku integration has previously relied on using the Rails plugin system, which has been removed from Rails 4. To enable features such as static asset serving and logging on Heroku please add rails_12factor gem to your Gemfile.