By placing the appointment of the Twelve immediately after the controversies with the Pharisees (6,1-11) and the dramatic distinction between old and new (5,36-39), Luke presents the appointment of the Twelve as the constitution of a new nucleus for the people of God, perhaps in deliberate succession to the twelve tribes of Israel. The conflicts between Jesus and the scribes and Pharisees have already shown that they represent the old and that, therefore, they are no more fit for leadership in the kingdom than old wineskins for new wine.
Luke makes special mention of the personal prayer of Jesus at all the important events in his life, and so Luke portrays Jesus as praying before his baptism, before his temptation, after a hard days work of preaching, teaching and healing and just before his choice of the Twelve. Jesus knows that even though humans will be weak and fail, even though they will deny and betray him again and again, he would still want them to collaborate with him in bringing about the kingdom.
The choice of the Twelve is a text that offers each of us a lot of hope and consolation. This is because we are aware of what Jesus could accomplish even with such a motley band of men. Since he did so much with and through them, he can do the same with and through us.