Grass clippings add fertilizer and organic matter to your lawn when left in place. When trimmed at 1 inch or less in length, clippings do not contribute to the buildup of thatch in the lawn, contrary to the commonly held belief that they do. When clippings are longer than 1 inch, they work well as a 1-2 inch layer of mulch around your plants to control weeds and retain water in the soil.
For more information, see University of Minnesota Extension
Photo by Susan O'Donnell