Deploying web applications used to be painful. Set up a server, configure Apache, install dependencies, manage databases, worry about scaling. It was complex and time-consuming.
Heroku changes this. It's a cloud platform that runs your Ruby applications with minimal configuration. Deploy with git push. Scale with a slider. Focus on code, not infrastructure.
Here's a challenge for your Rails development environment: Can your editor run a web server inside itself, capture runtime errors as they happen, and automatically jump to the exact line of code that caused the problem?
If you're using TextMate, Vim without extensive configuration, or a basic text editor, the answer is probably no. But if you're using Emacs with Rinari, this is exactly what you get.