When you delete a page, it doesn’t disappear from the search engine index. People can still find it, and land on it. When they do, they’ll see a 404 error. That’s a poor user experience, and can be harmful to your SEO. It’s really important to avoid those dead-ends.
The Yoast SEO Redirect Manager helps you to create and manage redirects, which stop users and search engines from getting lost. It even automatically creates redirects whenever you move or delete content. That’s great for users, great for search engines, and great for your workflow!