The preview circles are just a small thing, but very helpful to see where you're navigating to or what you're changing within the drag & drop editor.
Let's go to Sites and then to the landing page editor from there. Once you're in the editor and click on any element, you'll see small circles on the left side of each design element (header, text, background, links, etc).
When I click on the header and change the text color to blue, you'll see that the in the small circle preview next to Header settings, the color also changes to blue. This makes it a lot easier to easily navigate and find elements you want to change.
The small preview circles are always present in the editor, regardless of whether you're designing emails or landing pages.