We use Meteor because we were trying to find a really good framework for Node.js. We were originally building our applications with PHP and Symfony but decided to switch over to Node. We needed a framework that can be just as suiting as Symfony was.
We use Heroku because it offers a wide-spread of features that our studios love. We are able to deploy applications for our clients and send them out for testing, add in great addons, and even run domains on the platform.