Heat is radiated from the both the front and back of radiators, and that’s fine on an internal wall which will itself radiate the heat within the house. But on an external wall that increases the heat loss to the outside.
Putting foil on the wall behind the radiator helps reflect the heat and reduce losses. I’ve made my own from value kitchen foil and thick cardboard in the past, or you can buy a ready made product of coated polystyrene.
Product Climaflex Radiator Reflector Foil Roll Cost £2.80 per square meter; £7 a roll Supplier B&Q