Heroku is a Platform as a Service (PaaS), where you can deploy and manage your Ruby on Rails applications. It is one of the easiest platforms to deploy to; allowing you to focus on the development side of things instead of worrying about managing an infrastructure.
Sometimes we reach for a gem because it is what we know or we've had great experience with it in the past. However, sometimes the gem is a sledgehammer for a small problem. When you don't need all of the features, sometimes developing the functionality inside your application may be a better route.