There are two ways to do photography.
The first is to observe and record reality, the second one is to create it.
In the first case, the photographer’s task is to be at the point in space and time, where events are lining up in order to form the compositional solution.
This is how street photographers and reporters work.
The second way is to create a composition that, by its effect on the viewer, will equal or exceed the energy of reality.
That’s how studio photographers work.
In both cases, having “something unspeakable" in the frame makes it possible to turn the frame from a cultural phenomenon to an art object.
In other words, as Bart wrote, "when non-art becomes art".
Of course, there are not pure genres.
You can take a "portrait of a tree" or "dancing house", but anyway, it’s important to remember the "decisive moment" that Bresson was talking about.
The "decisive moment" is not an event that should be reflected in the frame, but the something unspeakable you saw, which makes your frame an event.