One reason is that the engine is horribly inefficient at low temperatures, and requires a very narrowly defined amount of fuel. (mechanics.stackexchange.com)
It can sometimes be very difficult to define the amount of coverage a person needs to be protected from accidents. (carinsurance.com)
At the protocol level, Bitcoin transactions define amounts in terms of this smallest possible unit (nicknamed a «satoshi»). (digitalcurrencycouncil.com)