The model of a system that already exists can be used during requirements engineering. It helps in clarifying what the existing system does. It can be used as a basis to identify the strengths and weaknesses of the existing system and hence can identify the enhancements required to be made.
It is not always needed for an existing system model to be complete and correct because it is not the model on which the entire requirements developing is based on. It gives the view of the existing system and the helps in identifying the new requirements. This model becomes a basis for the development. As the system development will not be completely based on the existing system model, it is not always needed to be complete and correct.
But it would not be same in the case of developing a model for new system. It is needed for these systems to be complete and correct. Models of the new system are used in requirements engineering to help stakeholders understand the proposed requirements. These models are used for the design and implementation of the system. As they models are used in the system development, these are required to be complete and correct.