A common issue that cause issues is when people create their emails using an old e-newsletter rather than creating a new email. Over time, enough superfluous code can creep in to cause confusion in the recipient’s email program, changing how the email is displayed. It also may be harder for you to create the email and you may get unintended results.
Also, and possibly more importantly, we sometimes tweak your template to make sure that it looks as it should in email clients/programs. If you use an old email you won’t be getting the latest version of the template.