I'm making the assumption that when you talk about microwaves you are talking about the cooking appliance. Liters are a standard unit for volume not just fluids. read more
Liters is used as a volume for the space in a microwave oven, backpacks, canisters , all sorts of things. *****Addition for clarity and completeness***** I can’t find anything definitive in relation to why liters are used for certain volumes rather than the SI unit of cubic meters. read more