The Apostle Paul rejoiced when Christ was preached (Phil 1:18) and said in 2 Corinthians 5:14 that the love of Christ constrained him to preach the Word. 

You begin with these motivations and then add that faith comes by hearing (Romans 10:17) and in I Corinthains 3:6 Paul says, "I have planted, Apollos watered; but God gave the increase" meaning our job is to sow and water the Word. 

You have the understanding then that man believes the Word because he hears it and since our ears work everywhere faith can happen anywhere man hears the Word. 

Thus, preaching Christ in the open air ultimately is done so man can hear and respond to the Word. 

The reason is that simple. 

The next step you take is the delivery of your message. Is it: clear, Scriptural, Christ centered, doctrinally correct, etc. Delivery of the message is where the preacher's study, prayer and preparation are on display thus the need for experience and training.

Ultimately preaching Christ in the open air is for the benefit and blessing of the hearer which leads to SFOI's constant mission to preach Him amongst fans and local citizens at major sporting events and why we have expositional preaching training and encourage all evangelists to be men of prayer and the Word. (Acts 6:4) and to aspire to the qualifications of an Overseer / elder (I Timothy 3:1-7) because they are preaching and teaching the Word.