The dry_run parameter is used to indicate that you are testing your Enhanced signals implementation. When this parameter is enabled, events will not be forwarded to Google Analytics 4, and thus, you will not see the events in the Google Analytics 4 interface.

The primary use case for the dry_run parameter is to ensure that the format of the events you are testing from Google Tag Manager (or your preferred tagging system) matches the expected format of the Enhanced signals endpoint.

You can read more about the usage of dry_run here.