The clippings return nutrients to the soil, so you need to fertilize less. They can also cool the soil and help it retain water. Do not blow or leave clippings where they will wash into streets or sewer drains and end up polluting our waterways. Clippings are high in phosphorus, a major nonpoint source pollutant. Keep your lawnmower blades sharp.
Dull blades tear the grass instead of cutting it cleanly.
The wounds cause the grass to lose more water, increasing irrigation needs or moisture stress, and also leave the plants more vulnerable to diseases .