Most of the time its probably not necessary. But if a topic is contentious, while one person is typing up a response to something already posted, if that OP goes back and edits their post, by the time the new poster submits, if the original post has been edited, it isn't clear what was being responded to. So as a courtesy, the person editing should explain WHAT they were editing. If it is a typo, rather than content, why not say that. For example, say I wrote:
"The grass is blue and the sky is gren"
And sat back to wait for everyone to chime in and tell me how wrong I am, and THEN edit my original post to say:
"The grass is green and the sky is blue"
Then all the people who posted in response to my original post, look silly. But if I go back in and say "Edited to correct typo" and they see that my OP now says:
"The grass is blue and the sky is green"
Then its clear that I am not backtracking but simply making it easier to read.
IMHO