I have gotten many shader requests for effects that shaders wouldn't be good at making. Today I would like to explain the uses of shaders so that you know when to use them and when to look in to other resources like particles.
Shaders are for changing the color of pixels. It isn't used for anything more. Shaders can use textures or math to mix colors or blend them, but shaders can't easily be used for clickable or interactive objects in a scene. In general shaders are for recoloring or distorting. If you need textured shapes that can have gravity or motion (like rain for example), then you should use particles.