The main reason is that if the removed products/collections/pages were already in the Google index, they won't disappear immediately - it may take a few days or even weeks before Google re-index them. So there is a possibility that the users may find removed pages in the search engines and when they click on any of them, they will be directed to the 404 page and the URL will appear on the list. But if you redirect broken pages to correct ones via our app, the users will see no errors.
Doc 404 - FAQ
