Here's a warning NOT to delete your Sierra installers! Apple has pulled it from purchase histories. Apple support document linked below - the line about Sierra is near the middle of the page.
To reinstall Safari, Mail, the App Store, or other apps that are included with the Mac operating system, reinstall macOS. macOS Sierra or later doesn't appear in the Purchased tab. Instead, use the Search field in the upper-right corner to find and open the App Store page for the current version of macOS.