The idea is that you change or remove your username after someone else starts a conversation with you, so the username can no longer be used to subpoena your account details.
Put another way, signal is able to provide those 2 pieces of information to law enforcement based on a phone number. This helps you to prevent law enforcement having a phone number to ask signal to look up in the first place, assuming you change your username every time you hand it out.
They also hash the usernames that they store on your account which means law enforcement can't ask what usernames are being used, only being able to ask for specific usernames which are currently in use.