publicly sharing your username attaches your nostr account to your signal identity.

"Usernames in Signal do not function like usernames on social media platforms. Signal usernames are not logins or handles that you’ll be known by on the app – they’re simply a quick way to connect without sharing a phone number. Your profile name remains whatever you set it to.

Your username is not displayed on your Profile Details page, and people you message can’t see or find your username without your sharing it. Put another way, someone will need to know your exact unique username in order to start a chat with you on Signal. And Signal does not provide a searchable directory of usernames."