Yes, but that can be achieved by using a hostname alone (router, nas, printer, htpc...). What does the domain bring?

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.`

In some cases today that hostname is being resolved via mDNS, which is a good reason to get into the habit of using .local for mDNS addresses. Using somehostname.local on many OSes today forces DNS lookups to always and only use mDNS and avoids accidentally calling DNS. (So long as you don't break mDNS support on your local network by doing something like trying to use .local for traditional DNS.)

mDNS is probably sufficient for most people and you don't need to setup old school DNS on a local network. (But if you do, a purpose-built TLD like home.arpa is a good choice.)

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.