When you train with firearms, things often happen at a fast pace. We use time as a metric, balanced with speed and continually push both of them. This can occasionally lead to our technique getting sloppy.
Or while making runs through a kill house, we can become almost overwhelmed with the amount of information our brains our trying to process. So much so that when we come out the other side, we can be hazy on what happened in the first room.
By incorporating video into our training we can see things that otherwise would go unnoticed or be forgotten. Whether you have your friend film you with a phone or mount a camera on your helmet or weapon, there can be a lot of value in the practice.