Why registration.php added in magento-2.0.0-RC?

registration.php is kind of the entry point of your module. It's the equivalent of app/etc/modules/[Namespace]_[Module].xml from Magento 1.
But now, is part of the module itself.
it allows you to create modules in the app/code folder and in the vendor folder as well.
No matter where you add it, this file will be picked up by Magento and your module will be taken into consideration.