Another reason for writing “edited” is that sometimes the meaning of the remaining post might be some what different than it would be with the “edited” part. So it’s important for the readers to realize that the post might not mean exactly what it sounds like it means (or at least the connotation might be somewhat different).
There were times when I was concerned about that fact with posts of mine that were edited (although usually nothing major; just a slightly different connotation), so I think it’s good that the word “edited” is there.
And I agree with those who wrote that there can be many different reasons for a post to be edited so it’s not necessarily something to be embarrassed about.