When you are customizing your theme, you want to be sure you got it right before you publish your changes to customers. The Preview button under the portal customization options helps you see how your theme looks on different devices.
This way, you get to work on your themes for as long as you need, without worrying about impacting the existing design. You can work on your layout changes, save them without publishing. You can publish the changes once you’re satisfied with the appearance of your portal across devices.