Generally, volume in litres is associated with liquid measurements. Space inside a car is a not a perfect cube or an easily calculable shape. Volumes are not always easy to measure for irregular solid objects or spaces. We can use litres and cubic centimetres, or even cubic metres for measuring solid objects, or space that something might fit into, for example, the space of the car boot, as well as for liquids such as drinks, water, and gas. This is the reason car boot space is specified in litres. It is also why the internal capacity of a fridge is measured in litres.