Natural use? As in, rain in nature? In nature, rain, the element which I shall soon be learning to manipulate, is the product of the water cycle. As water on the land evaporates due to the sun's heat, the newly-formed water vapour travels upwards into the sky, where the cold atmosphere cools the vapour into droplets of water, observed from below as clouds. As more and more water vapour evaporates into the sky, and then condenses into water droplets within the clouds, the droplets come together to make the cloud denser. When a cloud becomes dense enough with enough water, it will be too heavy to remain in the sky, and so the water will return to earth as rain, the rain adding to the sources of water from which the water droplets originally evaporated, and thus restarting the cycle. Sometimes, chemicals within the water as it evaporates, or even added as the droplets are within the cloud via gaseous means, can cause the rain to have certain properties, the most common being Acid Rain.
On the battlefield, Rain Release allows the user to, well, control rain. In a very similar nature as to how Water is controlled through Suiton, one may use this Ameton to control pre-existing falling rain. Much like Suiton, Rain Release may be fairly versatile, though it is restricted at the same time; first of all, rain must already be falling unless the technique causes rain. Second of all, while rain that's falling can be manipulated certain ways, it doesn't have quite the versatility a few other elements have; the rain can be collected into a water source, can have the rate of formation and falling altered or halted altogether, the droplets themselves can be reshaped to have different effects; to smash down on one like a hammer or cut through one like a sword, or possibly even changing the properties of the droplets, like can be done already with Suiton, for example, making it sticky. Of course, I may be selling it short on that point, but used correctly, versatile or not, anything can be useful.
As for anything else I know about Ameton, it is comprised of Suiton and Fuuton, water and wind, much like Hyouton. I know that I was able to prove myself worthy of learning this Release by demonstrating my prowess with those elements against one of my peers. From what I remember, it is likely that this release originated here, in Amegakure, due to the high amount of rain that falls and the need for ninja to take advantage of their surroundings.
I'm not sure if I know much more about rain...