Free mulch is falling right now, as the trees drop their leaves you can reap in a good source of organic nutrients for your garden. Leaves provide several uses:
Compost: shredded leaves can be added to the existing compost pile or be the beginning of a compost pile.
Leaf mold: leaves left to sit for a time will begin to grow mold. Adding these to your compost will help soil building by adding nutrients.
Mulch: mulch is very useful in a number of ways. It retains moisture, limits weed growth and in is useful in controlling the soil temperature.
Leaves absorb about 50% of the nutrients that the tree produces during the summer. When added to the soil or using as mulch, the leaves return these nutrients back to the soil. As an added benefit, they encourage worms and micro organisms to do their work to the soil. They work to lighten the soil which in turn allows the soil to help root growth and aids in the percolation of water. Good percolation reduces the compaction of the soil.
If you experience freezing temperature during the winter and you grow plants that can thrive during the cold weather, leaves can be used to protect them from frost and freezing. It will act as a blanket protecting them from the elements.