TTL uses a pre-flash because the camera usually does not know enough to calculate flash power from distance alone. It may only know an approximate focus distance, not the distance to every part of the scene, and it also can’t fully account for how the flash is being used—direct, bounced off a ceiling/wall, zoomed to a certain beam angle, or affected by nearby reflective surfaces.

Those factors change how much light actually returns through the lens. For example, the same subject may need different flash output outdoors versus in a small room with white walls, because the room adds reflected light.

Flash exposure compensation is still needed because TTL metering, like other reflected-light metering, makes a standard assumption about scene reflectance (roughly a midtone/18% gray). If the subject or scene is unusually dark or bright, the meter can be fooled, so you may need to tell it to give more or less flash than its default estimate.

So: distance matters, but it is not the whole problem. The pre-flash measures the real scene and lighting path, and FEC lets you override the meter’s midtone assumption.