It's a way to confirm you are you. It's easy for someone to copy your look, avatar etc. sure, your npub would be different, but an npub isn't human friendly.

I use onlynostr.club so when you see my posts and they have a tick next to mananguri@onlynostr.club you'll know it's really me and not an imposter.

Most of them also offer a lightning address redirect to your nostr name. That's why my lightning is the same as my nostr address.

It's free. Well, most are. Some charge a fee. Have a look, it's easy to setup.