Network security groups are used primarily to control inbound and outbound traffic based on a set of security rules. These rules determine which traffic is allowed to or from network interfaces in virtual networks. It is essential for ensuring that only authorized network traffic can access resources, thus providing a layer of protection against unauthorized access and attacks. Unlike VLANs which segment networks at layer 2, or firewalls, which inspect and filter traffic more deeply, network security groups operate at a basic level using simple rules to allow or block traffic.