A power rake works by scouring the layer of debris on the lawn using flails that spin at a relatively high speed. The debris is loosened and picked up by the machine to leave the surface of the soil much more exposed than before.
Improper lawn mowing and poor watering techniques can make your lawn start to form a layer of dead grass slowly over time. Soon enough, you will notice your lawn starting to form a brown color underneath the grass leaves.
This brown layer of dead grass and clippings becomes more prominent with time especially when you mow the lawn. This is when you need to power-rake the lawn because neglecting it for long will lead to an unhealthy layer of thatch in some cases.