Do I need to dethatch?  Thatch layers of 1 inch or more become barriers instead of benefits. Thick thatch blocks water, oxygen, fertilizer, and grass roots get trapped in thatch where they're vulnerable to heat, drought, and stress. Thatch buildup happens if there is poor soil aeration and drainage. Improper watering practices (usually too much or frequent watering, cold soil temperatures, the use of chemical pesticides, and the use of synthetic fertilizers are all factors that increase thatch accumulation in lawns.  Thick thatch also provides a breeding ground for lawn disease and insect pests. 

Power rakes are mower-like devices with rotating, rake-like tines that dig into thatch at the soil level and pull it up. Power rakes work well for lawns with thicker thatch layers and grass that can withstand intense raking. 

Overseeding?  Dethatching is a good idea prior to overseeding to expose more soil by removing the thatch layer. The tines of the dethatcher also cultivate the soil giving it a nice tilth which will help the seeds embed in the soil rather than just lay on top. This is similar to raking your vegetable garden to loosen the soil before planting seeds.  The dethather also helps groom the grass roots close to the surface. This will give the new roots more room to grow.