One small clarification that I found in general for deny all was
@DenyAll annotation can be used to restrict business interface access from anyone, logged in or not. The method is still invokable from within the bean class itself.
So the jist is it can be used for a method which is public for some reason or have been exposed (perhaps it implements an interface) but should never be called directly from outside. However they can be called from inside(within the class).
here is the link
One real example that I can give you is (which is quite related with my work). We have 2 business unit with same code base. Now in one unit there is a feature where some mobile reseller can directly call a service which cancels the voucher directly to the operator end but in the other unit we needed to block this due to some business rule. Since we use the same interface in both system so in one system we blocked its usage using denyall
Hope this gives you a clear idea.