A custom detection script would offer additional flexibility to determine if an app/package is already installed (beyond the built-in detection rules for app bundle/package receipt ID that are offered for Unmanaged macOS PKGs).

A specific scenario I recently had was the need to re-install a security app that was already installed, but switch to a different activation key (bundled in the package). With a custom detection script I would be able to check if the existing activation key on the device was correct, and if not return a non-zero exit code in the script to signal the package as "not installed", to cause Intune to re-install it with the new activation key.

There are other cases where the script may need to check for the existence of a particular file path of something else to determine whether it's already installed.