This has been mentioned before but I will try to paraphrase what is relevant.
Being able to edit your post is a good thing, However the intent of editing is to improve the content with newer and better content or to correct errors.
If your post has been replied to and changing the content would make the reply irrelevant or misleading then most users will often make the edit and with a following post will indicate that the content of the earlier post was edited so later readers can see what was changed and why. The reason for that is to keep a valid timeline as to what was said and when. This is similar and consistent with what is done in a verbal communication.
Deletion of posts or major changes can easily make the entire discussion of the topic on the forum disjointed and unintelligible. Similar to what happens when a person enters a verbal conversation and starts adding comments without waiting to fully understand the topic or where the discussion is headed. Their comments may be totally irrelevant to the topic because they do not have the full picture at the beginning.
Posts should be on topic and as much as possible relevant to the then current stage of discussion while later deletions or editing of the initial content is disruptive.
You may note that many times when a post is edited there is the word EDIT added with the new/changed content to make it clear what was done. Some even go so far as to strike out the erroneous or irrelevant content and add updated info â yet very few totally delete their posts.