Most are entirely ornamental and therefore serve no purpose. The bridge, in contrast, is certain to be quite functional. It is the safe and dry way to cross a stream or ditch. It instantly becomes the focal point of a path or perhaps an entire area. Once the location is determined the biggest decision is in choosing the materials.
Wood is practical and easy.
Stone is somewhat difficult but it will last forever. The rocks are cheap and the mortar is fun. While building, it is also unlikely anyone will steal your materials, even ifyou leave them out in full view as the project drags on for weeks. As far as the basic design, no one has done it better than the Romans. Anyway, a suspension bridge is a bit excessive for the back yard.