The family and friends should have the opportunity to make their needs known and to voice their anger and frustration with the alcoholics behaviour. By allowing the family to express their hurt an intervention can be therapeutic to everybody involved.
Interventions are widely regarded as being an effective means of persuading somebody to enter into a drug rehabilitation (rehab) clinic. Even if the intervention fails and the addict does not decide to get treatment then at least the family can feel that they have tried their best.
Whatever happens you can rest assured that you have tried your very best to move things forward and help the addict find a suitable addiction treatment centre.