See [My app has to be published first? But it's still under development](/questions/4573/my-app-has-to-be-published-first-but-its-still-under-development).

In order to use the API's write access, the app's owner must have an appropriate post that is legitimately tagged with [tag:app] or [tag:script].  
But this is a problem for apps that are still under initial development and not ready for general use.

The placeholder tag is thus the one time we tolerate an incomplete post -- trusting that the developer will complete the post properly once the app is ready for the public.