Throughout the spring and summer, grass clippings can coat the exterior coil of your air conditioner. In addition, leaves can coat the bottom and side of the air conditioner during the fall and winter. This ultimately hurts the air conditioner in two ways: it restricts air flow and it can rust out the bottom of the unit.
Regarding air restriction, the condenser coil must be clear of obstruction to maximize heat removal in the summer time. A clogged coil will increase your utility bills and shorten the life of the condenser fan motor.
The second concern is premature rusting. Corrosion can occur when leaves sit in the bottom of the AC. A sitting pile of leaves can remain wet for a long period of time in the fall and winter. Seasonal removal of these leaves will allow your AC to dry out as needed.