Email+ is a way to define reusable templates and components to be used in the OOB email notifications in ServiceNow. By putting everything in to a reusable template, the cost of maintenance will go down and the ease to change small things in a header across the company will rise.
Further more, Email+ also has:
- A feature to incorporate an email tracking pixel in to the email body, to track if an email has been opened.
- A feature to wrap the links in an email with a trackable link, so you can track the click rate of an email.
For further reading on how to use this, please se this article.