This is from Naomi Klein's Doppelganger:
"In The Origins of Totalitarianism, Hannah Arendt described the process of thinking as a form of doubling, because it is a “dialogue between me and myself.”
When each of us thinks and deliberates, we are in dialogue with the “two-in-one” that is our self, a self that, unlike a brand, is not a fixed, singular identity, or else what would there be to think about—or with? Dr. Richard Schwartz, who developed the therapy mode of Internal Family Systems, suggests there are actually more than two parts in our selves: every self is made up of a multiplicity, or mosaic, of often contradictory voices, hopes, and urges.
In extreme cases, when those parts become disassociated from one another, this becomes a pathology—what used to be called multiple personality disorder. Most of the time, however, the capacity to have an internal dialogue (or roundtable discussion) with the various parts of ourselves is healthy and human.
Moreover, for Arendt, it is when everyday people lose their capacity for internal dialogue and deliberation, and find themselves only able to regurgitate slogans and contradictory platitudes, that great evil occurs.
So, too, when people lose the ability to imagine the perspectives of others, or as she put it in her essay “Truth and Politics,” “making present to my mind the standpoints of those who are absent.”34 In that state of literal thoughtlessness (i.e., an absence of thoughts of one’s own), totalitarianism takes hold.
Put differently, we should not fear having voices in our heads—we should fear their absence."
Much, much more prosaically, one of the things that makes a good communicator is being hounded by the voices of other people in your head.
When I write things to be understood (ie: not this, but an email, or a presentation, or something) I can't help but imagine the voice of the person receiving the thing I've written. My eternal question is 'if I say this, what will they say?' And it surprises the heck out of me that other people don't do this.