Heroku is really just AWS with convenience, isn't it? I'm not sure what else they do - but I guess that "convenience" is worth it for some.
Once you move past that, because your app is well structured you can easily deploy on your own hardware.
AWS is the electric company. Heroku tends to be the circuitry, outlets and light switches for your house. You may take it for granted because it looks so simple, but it's the simplicity that makes it great.
As PaaS providers begin to mature and turn attention to more worthy challenges (like geo-agnostic deployments, easier scaling, for example), then their value is only going to increase.