The reason for carrying the travel voltage converter is that the voltage supply in America is rated at 110 volts whilst in many other parts of the world it is 220 volts. An American would need a travel voltage converter to step down the voltage in the new country to 110 volts so that he/she can use his/her electronic items. Likewise, a step up travel voltage converter is required in the USA to bring the 110 volts supply to 220 volts for use in electronics rated at 220 volts.
There are very many types of travel voltage converter kits in the market and therefore you need to know the wattage requirements of your electronic device and also the voltage supply of the country you are travelling to before purchasing one.