Subinterfaces are used to split a single physical interface into multiple logical ones. This is particularly useful in managing VLANs on routers and switches, enabling traffic from different IP networks and VLANs to traverse the same physical infrastructure without interference. In contrast, a VLAN tag helps identify network frames belonging in specific VLANs on a network, a gateway enhances communication between different networks, and a loopback adapter is a virtual device mainly used for testing and system configuration purposes.