The answer: so Google can find it, index it, and therefore boost your searchability.
Sure you've got a weekly (or daily) audio or video Podcast. And you spend oodles of hours preparing and researching and recording and editing. The end result is still just an audio file or still just a video file. The problem is that a Google crawler looks for text and it can't see (or hear?) what's recorded on that audio or video file.
The solution: get a transcription of the contents of that Podcast and post the entire thing on the same page as the embedded audio or video file. Now the Google crawler can crawl and run amok and do its little indexing dance across all of the text of your spoken words.
Need a quote for your Podcast? Email me at itypefaster at gmail dot com. Want to set up a weekly Podcasting transcript and have the text available right away so you can post both the Podcast and the accompanying text simultaneously? Email me at itypefaster at gmail dot com and we'll set up a weekly appointment and file transfer session.