They increase the voltage to allow an appliance to work safely and properly.
Answered by Rupert, Electrical Safety Expert
In countries where the voltage is 100-127v and the appliance, or its power supply, is not dual voltage rated, you will need to use a voltage transformer or converter to allow the appliance to work safely and properly.
Both voltage converters and transformers increase the voltage, but some ‘electronic’ appliances such as CD players, TV’s, radios, computers, etc are not suitable for use with converters. If unsure, check with the manufacturer.