IComment is inherited from IMetaDataContainer interface which expose the appropriate methods to retrieve, set and remove the metadata. The main reason why you would use this feature is if you want to save additional info together with the comment which you don't want to be displayed to the user. For example you might want your plugin to also save the IP address of the machine from where the comment was added. This is not relevant for the translator but might be useful inside your app. You can find a discussion here about storing metadata at segment level which might also help to understand how you can use this feature. The only difference is that in your case is stored at a comment level.

IComment is inherited from IMetaDataContainer interface which expose the appropriate methods to retrieve, set and remove the metadata. The main reason why you would use this feature is if you want to save additional info together with the comment which you don't want to be displayed to the user. For example you might want your plugin to also save the IP address of the machine from where the comment was added. This is not relevant for the translator but might be useful inside your app. You can find a discussion here about storing metadata at segment level which might also help to understand how you can use this feature. The only difference is that in your case is stored at a comment level.