Many consumer apps need to display ads in order to generate revenue.
The most popular ad network for mobile apps is Google Admob, and there is a community-maintained Capacitor plugin ready to use:
npm install @capacitor-community/admob npx cap update
Then, follow the Capacitor Community Admob documentation for configuration and usage.
This community article is also a useful reference for practical applications of this plugin (though it is using an older version of the plugin).