Power is important. If the player fails to generate enough, the base will effectively shut down, leaving you defenseless in the dark. To avoid this, you must build more generators quickly. It is the room most likely to be attacked by Saboteurs, because once it is destroyed, many of your doors, traps, and holding cells inside your base will no longer function.
This room is made available to you at the point where your power usage exceeds 18, which is exactly the point at which the indicator turns yellow.