A reaction plan is the series of steps that you would take in response to a specified abnormal condition.
A reaction plan helps to minimize damage. It reduces the time between the occurrence of a problem and a stopgap.
While a reaction plan is important, there is one big shortcoming. The problem with a reaction plan is that it is a workaround. The implication is that you have identified a risk, but have not done enough to prevent it. It is far better to have a poka yoke in place than to rely on a reaction plan.
That said, in the instances where you don’t have enough resources or skill to permanently resolve a problem, a reaction plan is an effective way of dealing with the situation.