What will happen to enable External marketing in Google play console?

External marketing Turn off external marketing if you don’t want your app to be advertised outside of Google Play

Is free? Is there any benefit to enable this?

Solution

Its simple

Enabled : your app campaign on google ads will show to all google ads services + all its partner website and app

Disabled : your app campaign on google ads only show to google owned services (Play store, youtube, google search, gmail, etc)