If you have a content manager that likes to use inline images in the Body field of a Drupal site, you might run into an issue where they want an image inserted at the beginning of the field but don't want it to display in a teaser. Of course, you could tell them to use the "Summary" part of the body field to manually set the teaser, but some content managers tend to forget about that, and you're also dealing with an issue of entering data twice. No one wants to enter anything twice if they don't have to (even if it is merely copy and paste).
You could create a custom display formatter, but maybe you're not comfortable with PHP. Or maybe you're just pressed for time.
The Advanced Text Formatter module is exactly what you need to solve this problem. After enabling it, you'll get an option on text fields (like the Body field) to use this formatter. After selecting it, you'll get the following configuration details:
Now you can better control what gets pulled out of the Body field to use on your teaser. To keep that inline image out, just choose the "Limit allowed HTML tags" filter and make sure the <img> tag is not in the list of allowed tags.