Grass clippings versus thatch: Keep one, not the other
Thatch and clippings on your lawn are not one and the same.
Grass clippings, the portion of mowed grass, are about 90% water, so they begin to decompose almost immediately after hitting the ground. Left in place, clippings return nutrients to the lawn & soil.
Lawn thatch on the other hand is the dead grass. In layers of a half-inch or thicker, thatch blocks water, air and nutrients from reaching the roots and promotes weeds to grow plus helps foster grass to become deseased.
Many grass varieties in a traditional synthetic lawn-care system tend to build thatch layers quickly. Excessive nitrogen pushes out excessive top growth, but it limits life in the soil and therefore slows decomposition of grass clippings even more. The process of de-thatching, removes the thatch, which can then be gathered and disposed or composted.
Some thatch is common and acceptable in all lawns, but too much must be removed. The good news is that natural lawn systems that add life into the soil will rarely have issues with excessive thatch.