Leaves in the garden
Thanks for your question! Organic materials such as leaves, pine needles and lawn trimmings can do all kinds of wonderful things for your soil and garden! So the answer is a balancing act between needing to get these buried so trillions of microbes (that you don't see) can begin to break them down (composting) so they can feed the plants you plant in the spring. But they are also important as mulches, so winter rains and snow don't weigh down the soil, and push out air. So, tilling some under and protecting the soil on top with some accomplishes both goals. Then in the spring--after the soil is dried out--you can gently re-till them so the mulching material gets down and available to microbes then. Here is a link to a good Aggie Extension article (with pictures!) that explains the process.
Hope this is helpful!