Basically, we do this for you. Your password doesn't appear for security reasons. Why? It's a safeguard against someone else somehow secretly getting access to your account. By not showing your password onscreen or in the source code for the page, this possibility is completely eliminated.

When you check your external mail, Yahoo! looks up your saved password, but nobody sees it.

If you already saved your password in the external mail settings, you don't need to save it again. We've got it.