One use case I’ve seen professionally is that if you’re in a datacenter shared with other people, one could easily plug a laptop or change your switch ports or whatever and see your database traffic or whatever. Or in the case of the cloud, it makes it so nobody can snoop on your traffic at the router or hypervisor level.
I’ve seen VMs getting traffic they shouldn’t be getting, so even if you trust your provider, bugs happen.
On smaller, regular LAN, some devices are pretty innocent on their own but may have vulnerable firmware and become part of a botnet, which then can be used for attacks like ARP spoofing.
I’ve had a conference room IP phone with a public IPv6, from another country, that triggered CPU warnings. It was being used to crawl our website and it was hitting some heavier pages and was trying all sorts of known exploits.
On my own home LAN, I just have VLANs and SSIDs based on trust level, but for the most part nothing that would be sensitive. I guess you could copy all of my Linux ISOs.