Robert Reppel:

… smaller subsystems communicate via agreed-upon contracts (commands and events). Each has their own data store. Eventsourcing and the CQRS pattern are used to reduce coupling (and therefore the potential for side effects) to a minimum.

What makes the combination of event sourcing, Domain Driven Design and CQRS so attractive is that it can greatly simplify building software which keeps subsystems cleanly separated and independently maintainable as more features are added over time, akin to what we have learned to do for cars, spacecraft and toasters.