Do you want to add promotional material to your Magento system emails such as the order confirmation email, or newsletter sign up email.
What about if you want a banner in your order email that promotes a discount code for 10% off the next order? You could go and edit the email template and insert the information. But when you want to change the offer you will have to re-edit the email template to change it.
Here is a idea: use a Custom Variable. Create a customer variable >System>Custom Variables.
Create a new variable that you could call email_offers. And within this create your offer ñ you can use the plain text or use the HTML if you are comfortable with that.
Now insert the custom variable into position on the chosen email template and save it.
From now on the offer will appear on the email when it is sent out. What is more, if you decide to change the offer you do not have to re-edit the email template you only need to edit the custom variable ñ making life easier for you and reducing the chance of making a mistake.