Headlines belong in newspapers or, more accurately, on the part that's showing in the rack or the newsboy's hand. They have to grab people's attention and capture the mood; they must notice events that have the most importance, however subjective that may be.
Headlines should be less startling than informative; readers are hooked in by the subject matter and a promise of how it will be told that's implicit in those highlighted, capitalised or bolded words. There's no need to oversell with an inadvised display.
Their equivalent on magazines and journals - not all but a large subsection - are the encapsulation of contents that take place on the cover. Whether this is informational or a selling point or both, it helps to define the publication more than just the masthead would.
Newsletters are as likely to emphasise their main story as the other publications but there is an added weight to choosing one news item to run with when there are so many stories to cover each day. Even magazines that use larger type to denote their main article, can do so at a considered distance unless they are Newsweek or Time.