Virtual LANs (VLANs) are used on a network to segment traffic between different departments or groups to increase security and reduce broadcast traffic. VLANs allow networks to be segmented without requiring separate physical infrastructure. Network Access Control (NAC) enforces policies on devices that access the network, which doesn't inherently separate traffic. Quality of Service (QoS) is used to prioritize network traffic but does not segregate it. Subnetting can separate networks but involves more complex routing; VLANs are a simpler solution for traffic segmentation on the same switch infrastructure.