Often this phrase is used to preface that the next statement will be frank or blunt. Think of how these words might land on the listener:- It may leave them suspicious and concerned about what you will say next (albeit for just a few seconds).
Noone likes that feeling, even if for a few nanoseconds. – They may wonder if everything you had said up to that point was fully truthful.
These words are really just padding used by the speaker primarily to make themselves sound more important. Most of the time you’ll find that the next statement stands up on its own without this intro. Or maybe you don’t need to say that statement at all.