Why is oil often used in cakes, such as the Ginger and Walnut Carrot Cake, instead of butter? Wouldn't butter give a better flavour?
Most cakes need some form of fat as the fat will help to keep the cake moist and fresh. Fat molecules also coat gluten molecules in wheat flours, helping the cake to have a more crumbly texture. Butter is often used in cakes as it has a good flavour and it can also have air beaten into it, which can help the cake to rise.
However some cakes, such as Nigella's Ginger And Walnut Carrot Cake use oil, or a combination of oil and butter. Butter is only 70-80% fat and the rest is water and butter solids. Using oil in cakes reduces the amount of water in the mixture and can mean a moister cake with a tender crumb. Usually oil is used when the cake has a strong flavour, such as ginger, spice or chocolate, where the flavour of butter would not be noticed as much, though cakes rich in eggs also sometimes use oil instead of butter as the eggs can provide a lot of flavour as well. You may also like to try Nigella's Toasted Marshmallow And Rhubarb Cake, which uses oil instead of butter.