These allow you to heat your towel rail/radiator electrically when your central heating is turned off. Dual fuel kits are comprised of an electric element and a chrome plated T-piece; this allows the element and central heating valve to be connected to one side of the towel rail/radiator. All towel rails/radiators are designed to allow an element to be inserted into the right hand side of the radiator.
NB: pipe centres will vary when using a dual fuel kit, to maintain symmetrical pipe centres use dual fuel valves.