A traditional system maintains its state consistent with respective business rules. When queried, this system provides its current state only i.e. where the system is, and no information about how it got there.
A simple approach to track the systems’ state evolution (how it got there) is by maintaining history. However, this approach is limited to providing information about state changes only. Moreover, the record-keeping becomes burdened with process details and has to evolve with all the processes that affect the state change.