If you offer the option to view prices in different currencies, then you will need this option. There are exchange rates located in the admin section of your shop but they are static, while actual exchange rates are not. This means you could be losing money on some sales or overcharging your customers on others. This package will make it so the exchange rates are updated on a regular basis, thus ensuring the proper price is charged.
If the Cron option is selected, then an option will be setup so that this update is ran on a regular basis, like once a day or once a week - whatever you prefer. If that option is not chosen, then the code is altered so that it is ran when you first go into your admin section. If this is often, then either method is fine. But if you seldom open your admin, then the second method may not be the best choice.