Leaves are packed with trace minerals that trees draw up from deep in the soil. When allowed to remain on your yard, leaves feed earthworms and beneficial microbes. They lighten heavy soils and help sandy soils retain moisture. They make an attractive mulch. They're also a great source of carbon to balance the nitrogen in the soil. And they insulate tender plants from cold.
Allowing leaves to breakdown in the landscape is a great way to 'build soil'. Florida soil is made up primarily of sand. Sand, by itself, does not retain moisture or nutrients for long, as they leach through the sand quickly. As leaves begin to breakdown, or 'compost' they retain moisture, allowing it to be more readily available to the trees, plants and grass in your landscape. As they compost further they create beneficial, natural, nutrients and they're free!