So apparently a lot of animals use the leaves as cover during the winter, so mowing it may kill all those animals.

A thick layer of leaves may not look like much, especially when the color is drained to a dull brown. But chances are it’s actually teeming with wildlife.

As temperatures drop, fallen leaves and the soil underneath offer crucial habitat for all kinds of creatures, including salamanders, frogs and rodents. Shepherd noted that this shelter is especially important for the smallest animals, mainly insects and other invertebrates. That list includes cocooned moths and bumblebee queens, as well as those that might be less eye-catching to humans.

Data suggests that one square meter of leaf litter could house between 40,000 to 50,000 springtails, tiny arthropods that are easy to overlook but important to ecosystems. Trashing leaves takes away an important source of protection for these and other critters in the colder months.

https://www.pbs.org/newshour/science/to-rake-or-not-to-rake-expert-tips-for-eco-friendly-autumnal-lawn-care