Prevent collision with public domains. “ws” has an A record. It’s possible that your local Webserver also is named ws.

With all new gTLDs it’s easy to have a collision when someone buys the “router” TLD.

How, exactly, is that hostname being resolved, though? There is no magical protocol to resolve TLD-less hostnames in DNS. Your router probably adds entries for `hostname.dhcp-domain` but since `dhcp-domain` is empty, you end up with just `hostname`. No, go ahead and name a device `com` and see what happens. Your router will have a hard time resolving `example.com`. This is why you put your stuff behind apurpose-built TLD like `home.arpa.`