Suppose you have a meeting that includes company employees and visitors from outside the company.
In this scenario, your employees have 802.1X enabled supplicants but your visitors do not. By configuring a guest VLAN, when your employees log into the network, they are granted network access (based on their user credentials and 802.1X enabled supplicants). However, when the visitors attempt to log into the network, they are granted limited network access because they do not have 802.1X enabled supplicant. The visitors might be able to reach the Internet, but they are unable to access the corporate network.