Here's where things get really interesting. Whether you're playing by yourself or with others, your city (or cities, if you choose to make more than one in a region) can interact with and affect others nearby. For example, if you're building a city whose economy is built on tourism, you'll want to construct special bus terminals, ferry and train stations, and domestic airports so Sims from other cities can travel to your casinos, hotels, and other entertainment offerings.
Or, if you build an industrial town, you may want to export refined resources, such as electricity, water, or garbage removal. You can offer your surplus supplies to the global market, where other players can purchase them as needed--best of all, the prices of these exports will change depending on real in-game supply and demand data.