If your Mac blocks the install, go to System Settings > Privacy & Security and click "Open Anyway."
If you are running an older OS like Big Sur or Monterey and cannot update to the latest iMovie version, a standalone DMG is the only way to install the app.
Professionals often prefer keeping DMG installers on external drives to set up new workstations without needing an active internet connection or an Apple ID login.
Occasionally, Apple hosts standalone updates on their official support site, though these are usually "delta" updates that require a previous version to be installed.