By installing the Blocksy Companion you will have access to:
A growing library of starter sites that will help you to speed up the building process. See the entire list here.
Create a transparent header and set it conditionaliy on certain pages. Also get more advanced elements.
Activate a certain functionality and load it's files only when you trully need it. More info about extensions here.
Get some performance controls that will make your website load faster and act smarter - more info.
Be the first in the line to play with the new stuff we are cooking. Not recommended to use on a production site.
We are constantly improving the companion plugin, and will add more awesome stuff soon.