Used in a conversation, when you want to indicate that the previous statement you made doesn't actually matter and that the listener can
feel free to ignore the previous statement.
Also, if you use the word "this" instead of "that", then it could be used to
preface the irrelevant claim you're about to
assert.
"Just saying" is a synonym phrase of the same meaning.
Margaret: Which of those two women over there is
your boss?
Julie: The one
in the pink dress.
Steve: She's really hot! (Margaret and Julie turn to stare at him.) What? I'm just throwing it out there.