I edited my posts a lot of times. Here are the reasons:
1. I noticed some typos and grammar mistakes after being posted.
2. I re-read my own post and felt that the words I used aren't very appropriate or some of them might offend someone. I usually edited those off before someone post a reply to it.
3. I added extra stuff onto my original posts.
I have never edited my post to change my arguments (from yes to no). But then, I seldom post in the CE forum anymore...
And if I have any extra points to add to this post, I might edit it later.