It is possible to make a good garden without structures - but difficult. The very word garden means 'enclosure' and it is likely that the first gardens adjoined buildings and were enclosed by walls or fences. Correct procedure is to design the garden and then to locate structures - including the main dwelling.
The range of structures to be included in gardens includes roofed buildings, enclosing elements, glazed buildings, gazebos, fences, railings and bridges. Since they are small, they might as well be high quality. As Tunnard said, 'use only the best materials'. These may or may not be the most expensive materials.
The word 'shed' is thought to derive from 'shade' and to have come into use in the fifteenth century meaning 'a shaded place for storage'. Sheds were therefore functional rather than aesthetic structures. But in the twentieth century men began to use sheds as private sanctuaries. Then the books began to appear: Gordon Thorburn wrote on Men and Sheds, Rob Beattie wrote on 101 Things to Do in a Shed and Sally Coulthard wrote a book on Shed Chic . So the garden shed became a fashion accessory.