The main reason why most condenser coils are coated is that they’re located in a harsh environment. These environments can be natural (saltwater) or manufactured (chemicals), but in either case, a coating will lead to increased unit longevity, maintained performance, and efficiency.
Nobody wants a corroded condenser coil. That situation leads to rapid losses in capacity, reduced efficiency, and increased energy consumption. In seaside locations especially, the operating performance of unprotected condenser coils may decrease over 50% in a single year.