The idea is to avoid sending bagged-up fallen leaves to landfills. Instead, we're asked to leave them be, allowing them to naturally decompose over the winter into nutrient-rich organic matter that also shelters hibernating pollinators and other beneficial insects.
Done thoughtfully, leaving the leaves is one of the best ways to turn yard waste into free fertilizer, and that's good for your plants, the environment — and your wallet. But it's important to consider the types of leaves you're dealing with and where they're landing.
Whole leaves should not be allowed to remain on walkways, where they'll create a slipping hazard, or on the lawn, where they are likely to cause disease.