It's interesting that the main reason given in this thread for typing not writing is speed. I can understand that; I type rather than write and I type faster than I write. Another popular reason given in this thread is for the ease of editing, which I can also understand.
However I was listening to an author talking on the radio the other day (sorry can't remember who but it was a renowned author, just not one that I have read) and he stated that he writes longhand for a couple of reasons:
1. Because it "SLOWS ME DOWN." He finds his writing more considered and better constructed.
2. Because "I can't edit it as I go." He finds that the ability to edit typos, grammar etc. as you go along interrupts his flow and pulls him out of the story.
I can see both of his points. I have tried to discipline myself to just type like I was writing. No looking back and correcting the paragraph that I've just written until sometime (much later) and I fail every time. When typing I just can't resist correcting a spelling mistake if I spot it, putting in a comma over there, moving that sentence to another place, etc.. This author argued that all that is the job of editing and should be done later; maybe when transcribing the written text onto your computer.
From my Essential Guide to Writing:
For drafting... keep going and don't worry about small mistakes. A draft is not the end product; it is tentative and imperfect. Writing becomes impossible if you try to do it one polished sentence at a time. You get lost looking for perfection.
He goes on to suggest writing by hand and then transcribing is a better way of doing it. Though he is also carfeul to say, "But this is advice, not dogma. People vary enormously.... Do what works for you.