There is room for those types of prefaces on internet forums. You use them when you are giving somebody "tough love" advice, and are genuinely trying to help.
For instance ... hypothetically speaking:
If someone came on the forum with a rant about a terrible experience you could reply with:
"Don't take this personally, but if you changed the tone of your posts you might get more helpful answers."
Now the preface lets the poster know your advice is not about THEM but rather about POSTS. Without the preface, the intent of the advise is muddied up by the personal nature of the statement.
It is no guarantee they will see the advise as you intended it, but they might indeed PERSONALLY be an apple-hole, and you can't do anything to help that.
Now if you are being a jerk, but are using it just to make yourself look less like a jerk ... then you're just being a bigger jerk.