A few fallen leaves on the lawn are generally not a serious problem. Wind may scatter them, while the remaining leaves can decompose and return nutrients to the soil. The situation changes when the grass is covered by a thick, continuous layer.
A deep carpet of leaves blocks sunlight and restricts airflow. It also holds rain, dew and condensation against the grass, creating a dark, damp environment where fungal diseases can develop. The original report specifically warns that prolonged coverage can contribute to snow mold, brown patch and leaf blight.
Fallen leaves can also attract pests. As the leaves provide shelter, warmth and moisture, they may become a suitable hiding place; once the material begins to break down, it can also provide food.
The leaves should be removed when they form a deep pile and the grass is no longer receiving enough light or drying properly. Raking is one option, while a leaf blower or lawn sweeper can reduce the physical effort and save time.
Mowing is another alternative when the leaf cover is light. In dry conditions, mowing chops the leaves into smaller pieces that can act as organic mulch and return nutrients to the soil. Wet leaves should be cleared where possible, however, because they can suffocate the lawn, encourage disease and clog the mower.
Whether raking or mowing, the recommendation is to work in dry conditions when possible. A light scattering can be left temporarily, but a heavy layer should not remain on the lawn for an extended period.
Sources: Tom's Guide.