Why clean up leaves? This may being going through some minds as we start to approach the leaf clean up season. Why even bother? What are the leaves on the lawn going to hurt?
The leaves are going to hurt the lawn by killing it. Not on a chemical level, but the leaves will shade out the grass, starving the grass of the sunlight that it needs to grow, and thus killing the grass.
Now this process of shading the grass out does take some time, but once the lawn is starting grow again in the spring is when this is going to be a problem. So we are talking late April to early May.
But why take care of them in the fall? Because it is easier to clean the leaves off of the ground because the snow has not had a chance to smash the leaves into the ground yet; they are still able to move freely.
So on a short answer to the question: Why clean up leaves? It is better for your lawn. And on a pure aesthetic level, it just looks better to not have the mess on your lawn and to not have to think about it all winter long.