Overseeding is essentially the planting of new grass in existing turf. It is an amendment that is used for thickening up a lawn- this is a maintenance procedure and is not the same as planting a new lawn.
Grass is like any other plant and all of us- it has a lifespan and will die out. If you do not replenish the grass with new seeds you can get thin, bare spots which are havens for weed seeds. Overseeding corrects this and allows new grass seeds to germinate in existing grass areas.