Today, there are 175 countries that have a 50Hz frequency and 45 other countries that use 60Hz. Within the two frequency groups, there are numerous voltages:
For an American manufacturer of electrical equipment, including home appliances designed to operate at 120v 60Hz, this difference in voltage and frequency has severely restricted their ability to compete in the global marketplace without a costly and fundamental dedication to manufacturing goods that meet each countries specific electrical grid specification.
For owners of electrical equipment that are exclusively designed to operate at 60Hz, moving and relocating to a country that uses 50Hz as its electrical grid frequency has been an insurmountable barrier.
Unless specified by the manufacturer to operate on both 50Hz and 60Hz, appliances may not operate efficiently or even safely if used on anything other than the intended frequency. Not only do they operate incorrectly and frequently face catastrophic failure, the clocks run slower while the motors run hotter and have a shorter life span. A regular transformer that changes only the voltage does not address the issue of frequency, the root cause of those problems.
Same issue exists for European or other manufacturers whose main markets are operating at 50Hz frequency. Also, owners of electrical products that are designed exclusively to operate at 50Hz, face failures, when they are used in 60Hz countries.