The theme editor lets you easily view how your theme will look like with other sites that you have access on.
To view your theme on other sites:
1. Click on the Previewing drop down box at the upper right
2. Click on the slider button for Search all sites
3. You can now view all the sites and you can also search for specific ones using the search bar
4. Just click on the site you want and you'll see your theme applied to that site