Why not just audio?
Simple audio is often the most effective media type that can be used. It is also the easiest to create and distribute. There are many excellent reasons why one might want to use video or a screen capture of a demonstration. We often make use of both in these tutorials and posts. Many of the screen capture files included in these tutorials do not include audio; but they are not attempting to convey deep understanding. They are just demonstrating buttons on a screen.
There is a reason why the simple audio podcast continues hold popularity over enhanced and video podcasts. ; Audio is simple and cheap to produce. More information and a deeper analysis can typically be conveyed to the listener. It is easy and cheap to access.