I agree, but this is only practical if you're dealing with a small amount of leaves. When you have a small army of fully mature maple trees around one's yard and you decide to just chop it up with the mower, you end smothering your lawn with a layer of leaf mulch. It's a mucky mess in the spring and it prevents light from reaching the grass underneath. Additionally while yes leaf mulch will biodegrade, it does not do so on a fast enough timeline. It can take an entire year if not more for this layer of mulch to completely breakdown.