One of two things happen for me: either I jot something off very quickly and then realize it has a bunch of grammatical errors or I forgot to say something, etc. Then I have to go back and edit.
Alternatively, I start writing and then get sidetracked. By the time I come back to the post, the thead has moved on, so I just delete what I was going to say.
Edit: just had to edit something! See what I mean!