New staff get to see the job done by an expert with a voice-over pointing out the critical points. They can watch it as many times as they like before tackling the job under the direction of a supervisor.
If you have an immigrant workforce, providing initial training with voice-over in their mother tongue can be a huge benefit. They may not understand everything they're being told by a supervisor who doesn't speak their first language, but they are much more likely to "connect the dots" on the job if they have seen the video of the task and heard a description they thoroughly understand. Costs are reduced if you have staff who can contribute to the translation and provide the actual voice-over in the appropriate languages.