Heroku charges $0.05 / hour (roughly $35 / month) per dyno. Many applications on may only require a handful of dyno hours per month.
For example, let's say that you have an application that sends emails in the background, and you need about 10 hours worth of background processing time per month.
10 (hours) * $0.05 = $0.50 at the end of the month
This is the amount you would be paying if you were only paying for what you were using. However, is not the case. Instead, you pay the full $35, if not more.
HireFire solves this problem by dynamically scaling your dynos up and down based on their current workload. If there aren't any jobs in the queue, HireFire will shut down all dynos rather than letting them idle.