By default, when creating a new article its permissions are inherited from the module. Anyone who has permission to view the module can view the article.
It’s easy to define permissions on each article. Simply uncheck “Inherit Article permissions from Module” and define custom permissions. Let’s say you’ve set the module to be viewable by everyone so you can show introduction of the article and then prompt the user to sign up or login to view the article. See: Marketing Paid Content
In that case, it would be tedious to specify the security roles every time you create a new article and hence the need for Default Permissions.