A Drug and Alcohol intervention is designed to make the alcoholic or drug addict realize the extent of their problem and how it effects those around them, particularly family members an friends.
Drug addicts will frequently lose sight of the extent of their affliction as they normalize something which is anything but normal. They rationalize their addiction and lose sight of the hole they are digging themselves into. The intervention process is designed to help give the addict perspective so that they can acknowledge that they have a problem and enter treatment.
An addict cannot be forced to get help barring very rare and extreme cases, so it is important that the individual be convinced of the gravity of the situation and the need to seek help. The abuser will be presented with evidence of how their behavior affects them an those around them. This will help motivate them as well.