Though there are many more reasons, here are 15 defenses of street/open-air preaching I wrote out this morning in response to a brother's request:
1. Street preaching is the foremost method God has used throughout the Bible to communicate His Word (our Lord and all the apostles and prophets were street/OA preachers; probably all five of our Lord's sermons recorded in Matthew were in the open-air (at very least the Sermon on the Mount in 5-7, the Sermon on the Parables of the Mysteries of the Kingdom of Heaven in 13, and the Sermon on the Mount of Olives, 23-25, which are explicitly said to have occurred in the open air), most of the recorded sermons of the prophets and apostles were open-air sermons.I will add to this that, biblically speaking, when you are talking inside a "church house" (or a house church), that's not preaching, no matter how loud you talk or how much "authority" you project. Preaching is, biblically speaking, what is done out in the culture to lost people. So I would edit Point #7 to "whereas teaching in a church house" or "whereas lecturing", as well as fleshing out the very real differences between "reaching" someone with a lecture among Christians vs reaching someone with the Gospel in an open air preaching context.
2. Street preaching turns the world upside down; it makes the church's presence felt by the world and demands a response. It is the most controversial form of evangelism and thus cannot be simply ignored by the world. It is, by its very nature, a publicity stunt.
3. Street preaching is one of the greatest antidotes to lukewarmness; it challenges our faith like no other spiritual exercise.
4. Street preaching is the form of evangelism most consistent with the definitions of the Hebrew and Greek words for "preach" (קרא; κηρύσσω), "evangelize" (בּשׂר; εὐαγγελίζω) and "boldness" (παῤῥησία) in the Bible.
5. Street preaching, because it is the bravest, boldest, and most intrepid form of glorifying the Lord and spreading His Word, is thus the greatest antidote to being ashamed of the Gospel or confessing His Name before men. It literally glorifies the Name of the Lord the most, in the most rigid sense of this concept -- God's glories and Name being trumpeted on the rooftops.
6. Street preaching invites the opportunity for faith-refining persecution like no other form of ministry.
7. Street preaching is the most aggressive, most militant, and fastest method of world evangelism and thus hastens the return of our Lord; whereas preaching in a church house can reach a 100 in a year, street preaching travels a 100 souls per hour.
8. Street preaching is more authoritative than one-on-one evangelism; its sermonic and rhetorical nature ensures that those who refuse to listen the gospel in a dialogue must hear it from the monologue of street preacher.
This is not a great point, as it trades on a dubious definition of the word "authoritative". Take that way, though, and it's a good point.
9. Street preaching is the form of evangelism most complementary to the doctrines of grace; only a true Calvinist with a high view of God recognizes the win-win scenario of God being glorified in the salvation of the elect and in the hardening of the reprobate to the vindication of His justice.Amen!
10. Street preaching makes the presence and power of the church known in the community like no other form of outreach, as it is most literally fulfilling the commands to set our light upon a lampstand, being a city set on a hill, and proclaiming from the rooftops what He has whispered in our ear.
11. Street preaching is the most inspirational form of evangelism; it is a double-edged sword in that it not only saves the elect outside the fold but greatly motivates and inflames the esprit de corps and morale of the army of God, the church. Many a lukewarm Laodicean Christian has been motivated to commit themselves to Christ, personal holiness, and world evangelism more seriously due to a street sermon more directly intended for the lost.
12. Street preaching is the most superabundantly fruitful form of evangelism. Ask any veteran field preacher.
13. Street preaching is the most manly form of evangelism; it takes most literally the command to "take dominion". Western culture has been emasculated, and sadly it seems as if the church has not been spared the effects of this neutering it seems. Now is the time for men of God to arise and to fulfill the command "Stand firm in the faith, act like men, be strong" (1 Corinthians 16:13).
14. Street preaching is the most angelic form of evangelism (the root word of evangelism being aggelos or angel); see Rev. 14:6.
15. Street preaching is safeguard against the erosion of freedom of speech; the day may soon come when America becomes a totalitarian state when the luxury of the constitutional right of freedom of speech is overturned by the SCOTUS. While you have the freedom to preach openly without the possibility of criminal prosecution or any life-threatening violent persecution may you seize the initiative!