Why should you remove it?  While a normal layer of thatch will protect the crown (visible part of the grass roots) of the grass plant and prevent soil from compacting, heavy thatch buildup will prevent rain and fertilizers from penetrating the thatch layer. This robs your lawn of the water and nutrients needed to grow a thick and healthy turf.

Mower clippings

Despite the common perception, allowing grass clippings to remain on the lawn does not contribute to thatch. Grass clippings decompose rapidly, adding nutrients to the soil. Clippings longer than 3 inches should not be left on the lawn, however, because they may smother the grass.