As a proper attestation that the username at that domain points to your pubkey for verification and that you control that configuration.

The majority of people wont do that, for the same reason the majority of people wont run nostr relays, bitcoin nodes, and will generally use custodial services as they have in the past.

Its not for everyone, which is why i noted "if you are able to". For people that want to be more empowered, more in control, more sovereign, its low hanging fruit