Almost everyone, native and visitor alike, will say Bang Fai are launched to bring rain, as in the Tourism Authority of Thailand link, below. However, a careful reading of the underlying myth, as presented in Yasothon and Nong Khai, implies the opposite: the rains bring on the rockets. Their version of the myth:
When the Lord Buddha was in his bodhisatta (Pali) (Thai: โพธิสัตว์, phothisat) incarnation as King of the Toads Phaya Khang Khok (Thai: พญาคางคก), and married to Udon Khuruthawip (Thai: อุดรคู่รู้ทวิป, Northern Partner-Knowing-Continent), his sermons drew everyone, creatures and sky-dwellers alike, away from Phaya Thaen (Thai: พญาแถน), King of the Sky)[3]. Angry Phaya Thaen withheld life-giving rains from the earth for seven years, seven months and seven days. Acting against the advice of the Toad King, Phaya Naga (Thai: พญานาค Phayanak), King of the Nāga (and personification of the Mekong) declared war on Phaya Thaen—and lost.