Bridges in a garden are serious additions and are amazingly simple either to construct or buy. The thing with bridges is this: they need to cross something. That they would be functional makes them even more appealing. They also are fun for kids to cross and they offer a great upright line to a typically flat landscape.
Add the construction aspect to it all, in terms of an appealing design, and you have a wonderful element to a nice and busy landscape.
There is also beauty in those bridges which are relatively flat, connecting land between larger bodies of water, for example. These can be composed of any number of materials, from cement to wood. There are some very nice plastic ones as well but I always question their stability in foul weather and wind. The wooden ones offer an amazing variety of possible styles and materials. They can be real garden art of the first order, complete with lighting and finishes to add depth to the material itself.
What to cross? I like any feature which deals in water, naturally. Now, while I use them most often to cross “literal” water, as in recirculating creeks and water features, they may also cross anything which changes a grade, like drainage ravines, indentations or connections between lawns. As I said, a “bridge to nowhere” jars the sense a bit too much. It would be best to find some organic postitioning for your bridge, with function an added plus.