Historically, speech has been privileged over writing.
Plato said writing weakens the memory, ruining true reminiscence and true wisdom.
Socrates said writing is static, etiolated, defenseless.
Rousseau blamed writing for isolation, domination, and inequality.
Bergson said writing is dull, empty, dead.
They all thought spoken discourse was the site of real meaning. Speech is the genuine, immediate, meaningful expression of interiority – the true way of bridging the fundamental isolation of one human from another. Writing is exterior, passionless, hopelessly distanciated from both reader and writer.
(I'm cribbing all this from Ricoeur's Interpretation Theory, btw, chapter 2: “Speaking and Writing”.)
We're living in an age where the distinction between speech and writing has been thoroughly blurred. (I dedicate all my Facebook chat logs to you, Derrida.) In fact, so much of our “spoken” discourse is now written or in some way recorded – on an almost daily basis I have conversations via text message and Facebook chat, and I don't even remember the last time I used my cell phone to call someone – that maybe it's a whole new category of discourse. People are discussing this, and I hope with a bit of time and effort on JSTOR and at the university library I can find a lot more stuff on the philosophy of internet-age discourse (recommendations are always welcome, friends!).
That said... writing is still my primary mode of discourse.
Writing isn't just the fact of putting words down on page/screen. It's a whole way of thinking. I write when my pen meets the paper and when my fingers strike the keyboard, but I also write when I construct passages of text in my head (whether they wind up actually written down or not).
In writing I can express myself without the impediments of physicality. In writing, no one expects me to decode all the mystifying subtleties of body language and facial expressions, and no one imputes to me implications I did not intend based on such subtleties that I was unknowingly expressing. In writing, no one will call me out for mumbling, or confuse me with an unexpected question, or look at me funny because I missed a social cue.
Writing is something I know. Writing is something I am comfortable with. Writing is something I feel I can control.
In writing I have time to pause, to reflect, to choose the right words. Writing slows me down, forces me to be self-critical – not, of course, that my written discourse is free of error and bullshit; but it's measurably more so than my spoken discourse. In effect, speaking merely voices my thoughts: writing shapes them.