Allows any entities that are referenced on a deleted entity's fields to also be deleted. This is the converse of Field reference delete whose purpose is to remove any references to a deleted entity from all your other entities (if I delete a taxonomy term, remove all the references to that term from my other content's fields).
The module also takes into account checking if a referenced entity is a default value for any field, and will not delete it in that case.
Commerce product reference and line item reference
Core taxonomy term, file, and image
Any field module which references an entity table and ID column in it's hook_field_schema() 'foreign keys' array.
Examples
An Entity reference field that is using the Inline entity form widget so that users can only create new referenced entities. If you delete an entity that has this field, then this can leave 'abandoned' entities that can no longer be referenced.
A taxonomy term field called 'Tags' which is free-form and allows users to add whatever tags they want. If you get a bunch of spam entries, you want to ensure that the tags the spammer used are also removed if no other content is tagged with those terms.