Most home and business Wi-Fi networks include at least one computer connected to the network's router through an Ethernet cable rather than via a wireless link. The wired connections are often in the same room as the router and modem, where there's little or no advantage to using wireless. In many networks it might be convenient to run a cable to one or more adjacent rooms but use Wi-Fi for computers in more isolated locations.
Other networks limit Wi-Fi connections to laptops, smartphones, and other portable or handheld devices. All the permanent systems use Ethernet cables, because they're faster and more secure.