Well, I'm new at this, but couldn't you just plug the Kindle into your computer and open the documents folder and remove all the books? I've removed a few at a time that way.
You could, but that means that the license for that particular copy may not have been released.
Amazon generally allows a book to be loaded onto only a specific number of devices at a time, usually 6. By removing a book the way you describe, as far as Amazon knows, the license is still in use so you're now down to 5. Do that a few times and you are clearly limiting your access. Now, if you only have 1 kindle, it's not a problem. But if, like many folks here, you have multiple kindles and/or multiple family members sharing an account, it becomes important to manage those licenses. Also, as you get new kindles and download the book to new devices, it's important that previously used licenses are properly released so that you don't find, when you get your 7th Kindle, that you can't download a\the book any more.
By removing via the Kindle either individually or by doing a factory reset, the licenses are, for sure, restored and available.