Let’s say that you have 2 laptops, 4 smartphones, and a tablet in your home, any of which could be connected to your Wi-Fi network at any given time. Let’s say you also have several smart home devices connected to Wi-Fi and a portable USB hard drive plugged into your Wi-Fi router to share files with other family members. Whenever a guest logs into your primary Wi-Fi network, they will also have access to everyone of these devices.
While it’s unlikely any of your friends would intentionally sabotage your network, there’s a chance that the device they are using to connect to your network is infected with some malware. If so, there’s a possibility that this malware could be spread to other devices connected to your network.