A speaker addresses a person for various reasons. For example, s/he may see that the listener is distracted and s/he may want the addressee to focus more on what s/he has to say. Moreover, it can happen in the middle of a speech, because what will follow is really important and s/he feels the need to underline it by asking for more attention from the listeners. Furthermore, the speaker may address a person because s/he may want to talk specifically to this person about something or in order to give him/her the stand.
So, from all the above, it could be concluded that the basic request of the speaker when s/he addresses a person is to grab the listener's attention.