Depending on your podcast format, narration may become a part of the final product (most common in Interview and Non-Fiction Storytelling). It may be sparse, simply setting the scene and introducing the topic, or it may reappear throughout the piece. Good narration allows the listener to make decisions for him or herself, but it focuses attention on what is important in the film. Bad narration gets between the viewer and what is on the screen and tells them what they should feel. Well-written narration can help to bridge the gap between scenes, adding to the speed of the transitions.
Many contemporary documentarists prefer to have the voice of actual participants narrate the story, but it is not unusual for the filmmaker to deliver the commentary, perhaps in a first-person account of creating the podcast itself.