The short answer is: everyone. Every missionary. Every person who has served in another cultural context for at least one year.
Because the most predictable aspect of missionary life is its unpredictability. Change is the status quo. While flexibility is an admirable and necessary trait, going with the flow can take people to places they never intended to visit. The accumulated effect of coping with the pressures of the mission field can result in stress fractures. Cracks that may go unnoticed if they are not looked for.
Coming off of the field–for any length of time–requires switching gears. That is an excellent time to check the transmission. And that, in a nutshell, is what debriefing is all about.