Explanation:

Statement 1: Buddhist monks went from place to place throughout the year, teaching people. The only time they stayed in one place was during the rainy season, when it was very difficult to travel. Then, their supporters built temporary shelters for them in gardens, or they lived in natural caves in hilly areas. As time went on, many supporters of the monks and nuns, and they themselves, felt the need for more permanent shelters and so monasteries were built. These were known as viharas.
Statement 2: There was no Buddhism when the earliest Viharas were made. Buddhism, as an organized religion, came into being after the death of the Buddha.
So, the purpose of preaching dhamma was a later purpose, and not the reason for establishing viharas.