In Cities: Skylines, as your city grows you'll notice people moving around the city - whether on foot, or in a vehicle. They move around for a number of purposes: residents look for jobs and families, tourists visit entertainment centres, and city services such as the fire and police departments will attend calls. There's even a supply chain where logs, coal, goods, etc, both raw and processed, are taken from one business to another (or exported!) as they get refined.