Videos are helpful to replicate the exact steps the player took before encountering the mission. Not only is this helpful for us to create very accurate reproduction steps, it also ensures we explore all of the numerous factors that might have led to this particular result.
These include but are not limited to; what skills were used, what gear, actions taken (grenades tossed, shots fired, movement etc), environment, other team players actions if in a group. All of these variables are almost infinite and very hard to narrow down, hence why videos are helpful.
Simply playing the mission could mean the team doesn't get the chance to explore factors that caused this particular issue to begin with 🙂