Tagging rules for application would be nice to have as well 😉 But for application it is not needed. If you want assign application to management zone, you can assign it to it using API and application name (or part of it). In such case tag will not be needed.
There are 2 reasons why we want to use tags.
1. Assign application to management zone by application name is difficult to maintain, because they can be changed sometimes. The tag we assign to applications are the application ids which doesn't change.
2. In their automation, all information are retrieved from their database including application id, but application names are not. Those names are just human readable references for the app owners to understand them.
For a very large and complex environment, we really need to have this feature to improve efficiency and maintenability.