How does the Readwise sync with Goodreads work?

You can connect your Goodreads account to Readwise, which will look at all the books on your Read shelf. For each book on your Read shelf, we will check if we have any Supplemental Highlights for that book. If we do, we'll add it to your Supplemental Books library, and you'll start seeing its popular highlights in your Daily Review.

This can help a lot when you're just getting started with Readwise and you don't have many highlights of your own, as you'll still be able to tap into the wisdom of books you read in the past.