Before any major escalation, the opening stages of an exercise reveal whether teams can recognise a problem, share information and organise the response.
Controlling the immediate hazard does not automatically restore normal operations. A useful tabletop exercise should test what happens around the response as well.
Emergency response plans usually define how responders reach the incident. A useful exercise tests what happens when that route cannot be used.
Emergency plans define responsibilities clearly. Real incidents still require responsibility, information and authority to move between people and teams.
Most organisations have someone who knows the site better than everyone else.