I could not put the following quotes any better myself:
"No sort of defence is needed for preaching outdoors, but it would take a very strong argument to prove that a man who has never preached beyond the walls of his meetinghouse has done his duty. A defence is required for services within buildings rather that for worship outside of them" - Charles Spurgeon
"Every great preacher of the Bible was an open-air preacher. Peter was an open-air preacher, Paul was an open-air preacher, and so was Elijah, Moses, and Ezra. More important than all, Jesus Christ Himself was an open-air preacher and preached, for the most part, outdoors. Every great sermon recorded in the Bible was preached in the open air: the sermon on the Day of Pentecost, the Sermon on the Mount, the sermon on Mars Hill, etc" - R.A. Torrey