We measure capacity because it's useful for everything. Capacity, for example, is how much milk do you need in a cake recipe. Without measuring the capacity of milk, it could be too liquidy or too dry.
For example, some people are very tight about what they put in their bodies to be fit. They limit the amount of sugar they drink and eat, but in this case it would be drink. They count every gram of sugar in the capacity of the drink.
