Facetracker is a free and open-source app that allows you to launch a face-tracking library on a Linux computer. To be more precise, it's a simple wrapper UI for the OpenSeeFace face tracker.

OpenSeeFace is a powerful realtime face and facial landmark tracking on CPU with Unity integration. It's a face-tracking library. There's an important distinction to be made here and it's that OpenSeeFace is not an avatar/VTubing puppeteering app.

There is not one singular answer. Even though VTubbing or streaming is not something traditionally associated with Linux, Facetracker is an app that's mainly meant to be used by users who require a local face tracker for VTubbing software.

However, this is not the only real-world use. This type of app can also be used by those who require face tracking for gaming purposes. One example that comes to mind is a racing/real-world driving simulation game. An app such as Facetracker can prove of great worth for peaking left and right in certain situations (without using any keyboard controls).

In terms of features, there's really not much to talk about. The app's simple GUI allows you to start and stop the tracker, and switch between available/connected webcams.

Users can also select a specific video mode (width, frames per second, height), choose a tracking model, as well as the option to set the IP and port (used by the tracker).

Despite its limited functionality, at the end of the day, Facetracker is without a doubt a very interesting app. As mentioned before, its real-world use cases are mostly tied to the user's creativity and needs. Of course, more features would have been great, but it's important to note that most of the app's functionality is tied to functionalities offered by OpenSeeFace.