I know you've seen those cartoons with the " one lump or two" line. Generally cubed sugar is used out doors at tea. It is premeasured, it is easier to transport and can't be spilled or blown away by the wind.
Sugar cubes are easier to use, offer a more accurate measurement and are easier to transport when you are traveling in the car. We we go on a road trip we use sugar cubes because they are harder to spill.