According to the BABOK® v3 guide, the purpose of defining the requirements architecture is as follows:

The requirements architecture is the way that the requirements are put together as a holistic view of the requirements for a change. It describes how different requirements artifacts (such as models and textual descriptions) relate to each other to form an overall requirements view for an initiative or change.

A business analyst would use requirements architecture for the following reasons:

• The requirement architecture defines which models are appropriate for the domain, context, and solution scope; for example, business process models to illustrate business processes and data models to describe data-related requirements.
• The requirements architecture also organizes requirements into structures relevant to different stakeholders; for example, the technology solutions department would want to see more detailed technical structures, whereas the head of the organization would just want a high-level overview, knowing enough to feel comfortable that the suggested solution meets the business needs identified.

• The requirements architecture also illustrates how requirements and models interact with and relate to each other. Ultimately, the requirements architecture makes sure that the requirements are met as a whole and in alignment with the overall objectives of the initiative or planned change; for example, if a requirement would not benefit the overall objective or is in contradiction with the objective, this should be discussed and potentially removed.