A cubic km of sea water would weigh a bit north of a billion tonnes. You're right that large numbers are hard to comprehend, but being able to summarise them a

A cubic km of sea water would weigh a bit north of a billion tonnes. You're right that large numbers are hard to comprehend, but being able to summarise them and convert to other measures easily helps convey meaningful information. Saying you want to process a billion tonnes of something is immediately grokable as vastly different to wanting to process a million tonnes. Being able to immediately convert that into a conversation about processing a trillion litres vs a billion litres is similarly valuable. If I can process 1 tonne of water per unit time, then I know that the cubic km will take 1000 times longer than a billion litres / million tonnes.