slice seeding (also known as slit seeding) is preformed with a machine that make slits in the soil and drops seed directly into them for great seed to soil contact. If your lawn has trouble spots this can be a good way to get a thick green lawn. The slits allow water and oxygen to reach the grass roots and promote root developemnt as well as a good seed bed for new seed to take root.
3 weeks fully grown
Core aeration reduces lawn compaction and introduces water, air and nutrients into the root zone which provides deeper root development and increases the lawns density. Aeration is the best way to prevent thatch build up.(source Cornell University)