All songs must end, but not all songs must end with an “outro.”
In fact, if I had to bet, I’d guess that most songs don’t have an outro. But--those that do will certainly end with one.
From a structural perspective, that’s what an outro is: an ending.
But an outro can be so much more.
Outros are like intros in every way--except one: location.
An outro can be short. An outro can be long.
An outro can be soft. An outro can be loud.
An outro can add a lot to the story. An outro can add nothing.
But there’s always at least one thing an outro will always be that an intro will never be: at the end.
One of my favorite songwriting techniques is to reuse the pre-chorus as the outro. This works well for two reasons:
Familiarity. The listener will refer back to the pre-chorus, which can give (or not give) the story closure.
It creates structural contrast. The pre-chorus necessarily introduces the chorus earlier in the song. Now it leads to—well, nothing. This is a clever way to “trick” the listener by imitating the tension created earlier without providing the same release.
The outro might be the most underused part of the pop song, but it should always be at least available in a good songwriter’s toolbox.