Home appliances, like TV, speaker, lights, may work with both 50 Hertz and 60 Hertz, but some appliances especially with electric motor inside it, like air conditioner, fridge, can only work with 50Hz or 60Hz. If a 50 Hz motor working on 60 Hz power system, the motor speed will be 20% faster, then cause over temperature and shorten its service life.
We all know, most European and Asia countries power systems are 50Hz, 220V (or 230V, 240V), but American countries are 60Hz, 120V (or 110V, 220V, 230V, 240V). Suppose a UK (230V 50Hz power) family move to US (120V 60Hz power) with their home appliances, those appliances are all designed for 230V 50Hz power, but can NOT work (or not work well) in 120V 60Hz power supply. In this case, we need something to change 60Hz 120V to 50Hz 230V to ensure the appliances work well.
Then it comes to pure sine wave static frequency converter, with variable frequency variable voltage output.
Static frequency converter can change fixed frequency (50Hz or 60Hz) power source to variable frequency power through AC → DC → AC conversion, output pure sine wave power. At the same time, there is a step up (or step down) transformer built-in the frequency converter to convert 120V (110V) to 220V (230V, 240V).
Related Source: How to select a Frequency Converter?