It helps people understand scale, since there is only one other kinda similar economic machine, and it's China. The EU is too loosely coordinated to really compare.
The population of Queens and Brooklyn is one of the most densely populated areas on the planet. I will never understand people who use massively outlier-sized cities as meaningful comparisons to nations.
Queens and Brooklyn are not outliers on the global scale. Most Asian countries has similar population densities if not higher. It's really the small cities in Europe that are the outliers here.