Sunflower is chloresterol free as well, plus has a naturally high vitamin E content which gives it a naturally longer life. It does not carry flavours from one cooking to the next and can be burnt at a high temp for longer periods than other veg oils without breaking down.
To get a longer life from the oil - filter through a fine cloth to remove impurities. Make sure the oil is only filled to 1 third of the container or to the fill level if an electric fryer, to allow for expansion when you put items in to cook. Use the right temp for the item too.
Why use one? It is a simple method of d'ehyrdating' foodstuffs, which means nutrirional content is maintained (i.e. boil potatoes in water without their skin and you loose 70% of thier vitamin content) deep frying is quick and gives a good result when used properly.