According to the BABOK® v3 guide, the purpose of the Assess Requirements Changes task is this:

Ultimately the business analyst must determine whether a change that is being proposed to a requirement will increase the value of the overall solution. If it will increase the value, it is important to then determine what action should be taken to make this happen.

Let's consider a practical example to understand this task further:  

A company that owns an online store has a requirement to be able to do deliveries to customers. It was initially decided that full integration into another specialized delivery company would be the best solution. However, it was later decided by the stakeholders that the same delivery company would still be used but full electronic integration will no longer be required and that the orders need to be reworked in-house before being manually submitted to the delivery company for execution.

In this example scenario, the Assess Requirements Changes task needs to be performed in order to determine whether the proposed change to the solution will increase the value of the solution and if so, what action should be taken to ensure this is implemented.

When the business analyst assesses changes to a requirement or set of requirements, the following must be considered: 

• Does the proposed change align with the overall strategy?
• Will the proposed change affect the value delivered to the business or stakeholder groups?
• Will the proposed change have an impact on the time to deliver or the resources required to deliver the value?
• Will implementing the proposed change make a difference to any of the risks, opportunities, or constraints associated with the overall initiative?