the way they smelled and the way the world smelled when the knowledge was shared.

smell, of course, being one of the oldest and strongest senses. And one which incorporates a vast amount of knowledge. True, most of this knowledge is non-verbalizable, but that's true for many many forms of knowledge.

I can think of a number of other things which would also be lost by video, but smell was an obvious choice.

As for practical, let's say I was teaching you to cook, or to hunt, or to practice medicine. Several areas where smell gives really rich information.