"Fallen leaves are nature's gift to the soil, as well as significant worm-fodder, as Amy Stewart would tell you. Like any mulch, they will keep the frost from heaving plants out of the ground, a great killer of fall-planted things in my part of the world. It takes me only a few minutes to rake the leaves from my city yard into my flower beds. The usual recommendation is to chop up big leaves with a power mower first. I don't even bother with this step and still have had zero problems with rot and smotherment in my sandy soil."

Link.