You just completed creating a brilliant campaign and would like to send a test email to yourself before setting the campaign to run. You send the test and see that the email is addressed to Jane and not yourself (see example below).
The reason the test email is addressed to Jane is because when previewing through a test email or in preview mode within the email editor, the data displayed will be based on whichever customer is selected via the "Preview with sample data from" box.
If you would like to change which customer's data you are viewing, simply choose a new customer from the list. This feature is also useful when testing out an email with product recommendations; the recommendations will change depending on which customer's data you are viewing.