Why is a system suitability test (SST) performed in addition to equipment qualification?

Clear and correct analytical results can only be achieved if the quality and performance of the analytical instruments are guaranteed to remain constant. This is essential for the assessment of the quality of the product to be tested. If system suitability tests are available, the risk analysis required in the event of equipment failures can be limited to the period back to the last successful system suitability test and does not have to cover the entire period back to the last successful qualification.

[GMP Compliance Adviser, Chapter 14.C.15]