The voice-over function makes it possible for you to create a basic voice-over, automatically from your storyboard. This voice-over can be printed to your soundtrack, and you can also download it for other uses.
Here you can check out an example of a Storykit video with voice-over!
The use of voice-over helps you in situations where you need to have some accessibility functionality. It should not be a substitute for a voice actor to read speaker text when you are in need of such but could be used for giving sight or reading-impaired viewers a way to follow the script even on platforms where a screen reader may not be functional or when you can't use a sidecar script file for that screen reader.
The technology behind this is Google Wavenet, a state-of-the-art speech synthesis solution that often yields a better readout than screen reader technology.