Cupolas were originally invented to provide light and ventilation to large buildings. Cheap and easy to build, they are also very attractive, so naturally they grew in size, decoration. Today we don't build them anymore, as we rely on ugly noisy energy hogging AC systems instead. 
Some cities are famous for their attractive skylines, filled with cupolas (and similarly used towers) of all shapes and colors and sizes, like Prague.
If you want to learn how to build them, but don't have the confidence to go spectacular like with a Russian style onion dome, start with a bird house or bird feeder. The principle is the same even if the size and shape and material can differ.
