When publishers consider book proposals, they're always thinking about how easy and how expensive it will be to market the book to people.

Paid advertising is effective, but extremely expensive.

Getting press coverage is great, and less expensive than advertising, but you've got to pay a publicist who knows what they're doing, and even then, there are no guarantees.

But if an author has a strong platform, such as an email newsletter with tens of thousands of subscribers, or a popular website or YouTube channel with tons of views, then it's an incredibly effective and inexpensive way to market the book. The author has already established an audience who has shown interest in their previous content, so chances are good that they will show interest in the book as well.

Not the OP but they want authors with big social followings or email lists etc, because the main risk of a book is will it sell - and if you already have a big audience to market to, that risk goes down a lot.