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

Older question, but FWIW Rails 5 Apps do not require the rails_12factor gem. Per the README:

We worked with the Rails core team to make Rails 5 work on twelve-factor platforms out of the box.

There is also a snippet for migrating to Rails 5.