It may seem silly to spend your valuable time picking leaves off your car, but in the long run it might be worth it.
Most of the time, leaves and other airborne solids blow right off your car. A Chevy Cavalier traveling at 120 mph has no leaves whatsoever inside or out. Using the same example, the driver of a Chevy Cavalier traveling at 120 mph has no brains inside or out, but that's another story.
One of the few places that leaves accumulate is just under the hood, on top of the vents that allow fresh air to enter the cabin. Over time, these leaves can really clog things up! This can cause your air conditioning to work harder, your heat to blow slow and overwork the fan. On top of these things, the longer the leaves sit and rot under your hood, the more likely they are to make something yucky that can creep its way into the space you breathe in. As a final downside, if these leaves sit moist for long periods, they can start to eat away at your paint and can lead to rust. That's car cancer!
Thankfully, it's easy to clean off. In most cases you just raise your hood and either vacuum or hand pick the leaves off. Two seconds worth of work, and you've done a good deed for your car. Now kiss yourself on the cheek.