If the lights are dim, the driver will see the road worse and drive will be more dangerous. If the light is too bright, you will be dazzling the drivers of cars traveling in the opposite direction.
In some cases, the headlamps cannot be adjusted and the lamp needs to be replaced. Car headlights also need replacing when the headlamp adjusters are corroded and cannot be fine-tuned. Not only does the headlamp need to be adjusted over time, but the brightness of the bulb also decreases until there is no more concentrated light flux and no clear beam angle.