Networks act as a grouping of screens, similarly to how you can have multiple screens per site, except networks are not restricted to a single location.
A good use case for this might be to group your displays that are on similar types of land, such as creating one network for campus display, and another for road-side displays.
Networks are required in our system, so if you do not need to group your screens differently, then you can create a single "Digital Displays" network to associate all of your sites to.