I have over 17 years of HVAC system design and installation experience.
There are a lot of things that can go wrong with condenser units when it's left unprotected in the outdoor elements.
The coils in the condenser are designed for a certain efficiency level and as they get dirty or the cooling "fins" become smashed, bent or covered in dust, dirt and other debris your unit will begin to lose its efficiency and may not cool your home as well or at least as inexpensively as it once did when brand new.
The longer and harder your unit has to work to cool your home, the more electricity you use, the more expensive it is to run it.
It is very important to keep your outside air conditioner coils free of as much dust, dirt, leaves, bugs, small animals and other outside yard debris as you can.
Outdoor air conditioner covers will protect your outdoor condenser for a good 5 to 6 months during the fall and winter months and prevent just about anything from getting into your AC unit and disturbing the coils and operating system.
It also means that your AC unit will most likely avoid getting large amounts of snow and ice in your air conditioner, which could severely damage the coils, cooling "fins" and internal operating system.
Remember: water expands when it freezes, so a little water or snow on the coils during the day could quickly turn to ice that night.
The air conditioner cover needs to be made out of a high quality, long lasting breathable material so that condensation does not build up causing further problems.
Some so-called "experts" may argue that the outdoor condenser is built to withstand harsh winter elements so no cover is needed.
I believe differently based upon my experience and knowledge.
When I began selling air conditioner covers I worked for a nationally known HVAC company as the Sales Manager.
We were selling the covers to existing and new customers as a way to protect the customers' investment.
Our customers were thankful for our advice and we were selling a lot of covers.
When the corporate office heard we were selling the covers they were very upset and ordered our regional office to stop and decease.
Their reason? Because they were in business to sell new condenser units and/or repair them...
they didn't want customers protecting their units from outdoor elements that will harm the AC units or prolong the life of those units!