They use Gutenberg texts, but clean up the formatting and do another quality check review for weird OCR. They also make sure hyperlink chapter numbers function (not always present in PG files) and give the book an appropriate cover.
In some cases, they create omnibuses, too, like for Wodehouse.
And for every author/book they've done, there is a page with a description and what makes it interesting or worth reading, which is kind of nice.
It's mostly stuff anyone can do with their PG file, but it's convenient to have a website where the aesthetic things are already done.
