I don't know about most of their money, but almost all of the companies listed on their (fairly extensive) success page are start-ups or small businesses.

To me, Heroku makes the most sense when you are building a business that may scale very quickly, and especially one that does not have a full-time sysadmin. It also makes sense for web apps that will stay very small and unlikely to grow that much past the free tier, but those are pretty much a loss-leader for Heroku.