Halliday & Matthiessen (2014: 159):
In using a Vocative the speaker is enacting the participation of the addressee or addressees in the exchange. This may serve to identify the particular person being addressed, or to call for that person’s attention; but in many dialogic contexts the function of the Vocative is more negotiatory: the speaker uses it to mark the interpersonal relationship, sometimes thereby claiming superior status or power. The Vocative is also brought in as a text signal, for example, when signing off in a telephone conversation.