Reading through Acts 1 today, I stumbled across vs. 20 - a very interesting verse.
“For it is written in the book of Psalms, ‘Let his homestead become desolate, and let there be no one to live in it’; and ‘Let another take his position of overseer.’
Having looked at the Psalms to which Peter is here alluding, Psalm 69 and 109, I really struggled to see how this psalm had anything to do with Judas' betrayal. Looking through the commentaries provided no relief either. Until Bock, that is! Here he notes the typological-prophetic use of these two psalms, which explains the use quite well...
The psalms Peter cites (69:25; 109:8) are about the unrighteous or the enemies of God, who ultimately are judged. Judas belongs in this category, so Peter applies the text to him in what is called hermeneutically a typological-prophetic manner… The first text is Ps. 69:25: since the defection has occurred, there is a reference to the enemy’s house being left desolate. The second text, Ps. 109:8, refers to what needs to be done to replace Judas. Someone must take his place of responsibility.
The psalm [69:25] discusses the enemies of God. The psalmist cries to God to be delivered from them and calls for God’s judgement so that their camp is lef desolate and no one is able to live in their tents. Peter applies the psalm typically-prophetically to indicate that Judas has experienced such a judgement. They type of death Judas experienced left the field desolate for him and others. Matthew 27:7 notes that the field became a cemetery… The point of Peter’s citation is that judgement has fallen on this enemy of the righteous Jesus.
Once again the psalm  in the MT is a lament of the psalmist asking for God’s judgement. The request is that the enemy’s days may be few and “another may seize his position [or goods].” Peter also uses this text typologically-prophetically to declared Judas judged. Judas’ position is free to go to another. Scripture justifies the new election.
I think this explanation is very helpful. It still baffles me why Luke chose to record this episode. But perhaps Theophilus had some questions regarding "the Twelve". I find it strange that they disappear from memory in early Christianity... What was the point of having them in the first place? Why did Peter feel the need to keep the Twelve "intact"? I understand why the historical Jesus would want Twelve disciples, but early Christianity? It doesn't really make that much sense to me...  Bock, Acts, pg. 82  Bock, Acts, pg. 85-86  Bock, Acts, pg. 86