Why should the Survivors be promoted?
When the Survivors are promoted, their Food/Water consumption rate is decreased, while the combat ability is increased.
When promoted to 2-star, the Survivor's skill effect increases by 3 times. For each time a Survivor is promoted to a 3-star or 5-star status, one life is increased, giving a total of 2 chances of surviving zombie attacks.
An added benefit of the promotion is the increase of Parts gain amount by 100%.
In case of one Survivor, promotion should be top priority by collecting all the required Parts since powerful bonus effects are given.