Since not many of us can read lips, effective communication can not occur without intelligible audio. Audio delay can make interaction difficult. Audio that is not synchronized with video can be distracting. The value of video Intuitively, it seems that video adds value to a conference. Video enhances communication by creating a sense of presence.
Video allows for communication through gesturing.
Objects can be shown to other participants by holding them up in front of the camera. Video allows for interpretation of what is going on in the environment of other participants. Perhaps this is a side-effect of a generation that grew up with television. Most desktop videoconferencing applications have a shared whiteboard capability. In the lecture environment, it is very helpful to have a good view of the speaker's written materials.