The premium version comes with a few more features
Customize email content
No need to edit the template files, with the premium version you can edit the content directly in the settings page. Use the macros.
The enable/disable is there in case you do not want to overwrite the email template before editing the content field.
Multiple Subject and Headlines
To help prevent your customers ignoring your emails you can enter up to 5 different subject and 5 different headlines, each using macros.
You can leave any subject or heading field empty and they will be ignored if you only want to switch between a few different ones.
The premium plugin comes with some suggestions.