Outdoor AC units are durable and can withstand all weather conditions. However, covering your outdoor AC unit can be very useful; 

-Direct sunlight can decrease your efficiency and slightly decrease the cooling capacity. 

-Winds can cause tree leaves and twigs to get into your AC unit. 

-Water from heavy rainfall can cause damage by getting on the coils and freezing. 

By covering your AC unit,  you can also protect it from dust and debris while maintaining cost-effective maintenance.