Leaf mold is not the same as moldy leaves. The former is an excellent conditioner for garden soil and will add vital elements to it. The latter is likely some kind of plant disease.
Leaf mold is essentially fungus decomposing leaves. Whereas any number of microscopic bacteria may break down the leaves in your compost pile, leaf mold is a specific organic process where fungus breaks down the leaves.
You can use dead leaves as mulch in your garden to protect your plants from hot and cold temperatures and add nutrients to the soil. Also, by gathering leaves, shredding them with a lawn mower, and applying them to your garden, you prevent leaves from ending up in landfills and save money.
For years, I considered dead leaves littering my lawn a nuisance. Cleaning up dead leaves, loading them in huge plastic bags, and disposing of them was a staple of my fall. Little did I know, I was wasting an amazing free resource for my garden.