Why would you use a Guest network on your WiFi?
Well for guest naturally, as well as attaching IOT devices to a network separate from you usual home network.
If you have wireless devices and sensors, the guest wireless network can be used easily.
For example, the NEST thermostats can be attached to the 2.4 mhz Guest network if the ping and strength are OK. The Rachio irrigation controller also works here. Smartplugs such as TP-Link, work as well.
Smart speakers such as Amazon Alexa devices (echo, dot) can be attached to the guest network as well.