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The default attribute in a model field has exactly that purpose:
If an object has no watermark_image attribute use the default image.
Why is this important?
Imagine if you have a field that cannot be blank, because it would otherwise lead to a database inconsistency.
For example if you would have a User model that has a profile_picture field.
When looping through a list of users in a list like this:
[user.profile_picture for user in User.objects.all()]
When you have a default value set on the model level for the profile_picture attribute you do not have to worry about this code. If you would not have that default value this code could raise an AttributeError for users that have no profile_picture set.
Therefore it is impossible to delete an attribute that has a default value set in the admin, because it would eliminate the purpose of the default keyword argument in the model attribute.