When grass is mowed, especially if it is cut short and the soil becomes exposed, a significant amount of moisture is lost. Leaving the clippings on the lawn acts as mulch, helping to reduce soil exposure and retain moisture. As the clippings dry, a large portion of their nitrogen content is lost through various pathways, such as volatilization (release of ammonia gas) and microbial decomposition. Essentially, the clippings transition from being nitrogen-rich green material to carbon-rich brown material. However, most other nutrients, such as phosphorus, potassium, and micronutrients, are largely retained in the clippings.

My wife forces me to bag them because of her "allergies".