Rain Release is using Wind and Water to produce clouds in the air capable of producing rain. Rain that is produced is then manipulated via one's chakra in the same way Suiton us, reshaping it for various purposes. Using Water and Wind in the test wasn't very hard for me because they are elements I use a lot anyways, and also are very versatile, so any usage I needed was provided for me. The only problem I'd have with rain is how repetitive it gets as the jutsu list goes on. For me, it seems more like something where the best is brought out in customs