Why is it so important to be able to convert metric units of capacity?

It will make it easier for measuring

It will make it more easier for you, I know I sound silly saying that but it honestly does. In life you need to simplify things so that it is easier to solve.

For example, if I were to have 57 items in my pencil pouch, and you ask to borrow a pen, I won't state every single type of pen I have ... I will just give a basic pen.