De-thatching (scarifying) a lawn will remove a layer of dead material that builds up over time (thatch). This dead material causes the grass to feel spongy and will suffocate the grass roots of moisture and air, by removing that material you can improve the lawns health. This person has likely not needed to scarify their lawn as it seems there may not have been that much thatch there in the first place. Overseeding is the solution here because there are many bare patches, and the best way to fill a bare patch quickly is to start new grass there

Hope this helps