During the planning phase of an assurance engagement, gaining an understanding of how the area under review is accomplishing its objectives is crucial. Reviewing key performance indicators (KPIs) is a primary technique because KPIs are directly tied to the objectives and provide measurable data on performance. This review helps the auditor understand whether the objectives are being met effectively and highlights areas that may require further investigation.References:

The Institute of Internal Auditors (IIA) - Standards for the Professional Practice of Internal Auditing, Standard 2201 - Planning Considerations