Bridges and tunnels are used to overcome obstacles in nature. Such as crossing big rivers with bridges or crossing through mountains with tunnels. Some bridges may differ in how they are built. For example, there are bridges like beam bridges which are used for short spans of distances, Arch bridges which are used to support bridges when it is hard to them support them due to many obstacles in their way and lastly suspension bridges and they are used to withhold heavy cars, trucks for long distances. Building tunnels are hard too since engineers have to carefully plan the kind of soil or rock they need to dig through. If the tunnel were near water, they will have to suck the water out.  

Bridges and tunnels are very important because they provide alot to humanity to cross over long or short distances. They also last longer than most roads or railroads so they are very efficient too. They also let resources be provided to other places too since they connect eachother. Most places are connected with bridges and tunnels and you travel across them quite frequently but you just don't realize it.