Open air preaching was the primary method YahShua and His disciples used to spread the gospel in the first century. When reading through the four gospels, you will discover that YahShua was almost always speaking to the multitudes. In fact, of the four one-on-one conversations he had with people while ministering, three came as a direct result of an earlier sermon that he had preached to a crowd.
YahShua, like the prophets before Him, went to the market squares, the gates of the cities, even the river Jordan, wherever the masses congregated. He would lift His voice up at the major sea ports and draw massive crowds. On at least two occasions he talked to crowds that consisted of thousands of people. YahShua performed most of His miracles while preaching and in the midst of crowds.
Peter's first sermon was to a crowd so large, around 3000 people from that crowd repented and believed. Paul and the other disciples traveled from city to city, finding the most public places, lifting their voices and drawing huge crowds. In Athens Paul went directly to Mars Hill, a large university area where hundreds of philosophers would congregate to discuss ideas. Paul addressed those gathered there, which would have made up quite a crowd.
All through the book of Acts, the disciples preach to huge crowds which either respond by repenting and believing, or respond by rioting. Over and over again Paul and the others are thrown out of cities by raging mobs, stoned and left for the dead. These crowds did not come merely because the disciples held an ice-cream social to "minister" to their neighborhood. They didn't practice the modern church's popular "friendship evangelism" or the one-on-one approach at the local malls. Those are all valid ways of spreading the gospel, but simply don't have the effect that the original methods did (i.e., RIOTS or REVIVAL).
No, these mobs pursued the disciples because they consistently drew huge crowds when they preached.
Now that we have established the fact that YahShua and all of His disciples used open air preaching as the primary means of spreading the gospel, we need to address why this was true.
The method is very efficient at communicating information to a large number of people in a very short period of time. Because of the confrontational style of the method, it is also very good at forcing whole groups of people, towns and cities to either accept or reject the message, allowing the preachers to either continue their labors or wipe the dust off their feet.
The method is very cheap. It costs nothing to find a populated, public place, lift up your voice and begin to preach. Compare this to the untold millions spent on stadiums, conference halls and churches to put on elaborate evangelical crusades. The advertising, the buildings and the logistical support needed takes valuable time and lots of money.
With street preaching, no PR is needed, no advertising, no money need be spent save on the transportation it takes to get to the desired place. Campus preaching is even better. Since the 60's, most public universities have special areas designed for freedom of speech. Often these areas are in highly trafficked areas and come equipped with a platform and even benches for the listeners.
The amount of people you can reach with this method varies with the location and the type of crowd, but it is far easier to reach hundreds, even thousands in a few hours as opposed to one-on-one witnessing. Friendship evangelism nets maybe one or two people after several hours (or even days) of work. And we are not talking about converts, but the number of people who have been presented with truth. No wonder so many eager evangelists throw in the towel when they see how few people they actually reach using the church's prescribed methods.
