Imagine you work in a three letter agency and discover that a certain guy has a vital role in an issue you are investigating. You don't want to just bug his iPhone from now on, you want to know everything he said over the past two years when around his iPhone.

The tech is already included in the iPhone, and storage requirements are in the hundreds of Megabytes, not more. The device is a black box on purpose, and my informed opinion is that they have been logging the ambient text-to-speech since a long time.