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

Just ran into a problem with a Rails 4 application on Heroku and couldn't debug it with heroku run logs. Without the rails12factor gem, you can't see the full logs that you normally see on your console in development. I was getting a 500 error and there wasn't any available information.

It actually didn't tell me that migrations were pending without rails12factor