Corners receive less light than the centre for several reasons: the lens barrel physically obstructs off-axis rays, light strikes the sensor at a steeper angle, and brightness falls off naturally with the cosine-fourth law. The effect is strongest wide open and at wide focal lengths, and usually diminishes on stopping down.
Mechanical vignetting is the avoidable version — a hood, filter ring, or stacked filters intruding into the frame, producing hard dark corners rather than a gradual falloff. That one is a mounting problem, not an optical characteristic.
Deliberate vignetting is standard practice in post. A subtle darkening at the edges keeps the eye from wandering out of the frame and pushes attention toward the centre, which is why it appears in portrait and cinematic grading constantly. The line between atmospheric and heavy-handed is thin, and lens profiles will remove the optical version automatically if you would rather add your own.