The demons identified Jesus as the Son of God, and Jesus gave them strict orders to keep quiet about his identity (Mark 3:11-12). His family thought he was out of his mind, and the Pharisees used their words of doubt as an occasion to accuse Jesus of being possessed by a demon himself (3:20-22). The demons knew the truth, but didn't need to be speaking it. Jesus' family and Pharisees didn't know the truth, so their doubts and accusations were detrimental to the cause.
Somebody needed say something! And Jesus found the somebodies. He found twelve men and appointed them ambassadors. We usually call them "apostles." He called them so that they would spend time with him; and then be sent by him to preach, and to show that he was not possessed by demons, but had power over them (3:14)!
The doubters are still talking. Believers better say something! That's Life at Work!