According to the BABOK® v3 Guide, the purpose of validating requirements is as follows:

If you ask yourself "Am I building the correct solution?", you will ensure that you have always validated your requirements.

You will remember that in the previous section, where we addressed the verify the requirements task, we asked "Am I building the solution correctly?"

This is a simple way to differentiate between verifying and validating requirements.

Requirement validation is a task that is performed throughout the life cycle of the requirements and designs and is primarily concerned with ensuring that the requirements and designs remain aligned to the business needs. This includes stakeholder requirements, solutions, and transition requirements. Ultimately, the purpose of implementing requirements and designs is to achieve the business stakeholder's desired future state. 

The task of validating requirements is also performed to ensure that all stakeholders remain in alignment with what is required and it helps identify any requirement conflicts that might exist.