Depends on the zone.

In Texas where I lived… you get snakes. BIG snakes. And fun things like ninja bug swarms, wasp colonies, black widows, and such.

There aren’t a lot of wet leaves in Florida with so many palms or evergreens, but if you do leave them, you get disease and parisites. Like, all the native plants in your yard will start dying. And it’s definitely not safe to step in.