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Central element in the pattern is having separate data structures for reading and writing information.
CQRS is mainly used in complex domains where a single model to handle both reads and writes gets too complicated. Simplifying can be achieved by separating the models. Especially helpful in cases with a lot of reads and very few writes. However, it adds additional complexity of having separate models.
Source: CQRS by Martin Fowler. What do you mean by “Event-Driven”? by Martin Fowler.