What external auditor rotation achieves
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External auditor rotation requires a company to change its audit firm, or at least its lead audit partner, after a set number of years, rather than allowing the same relationship to continue indefinitely. The rule exists because familiarity between an auditor and the people they audit can gradually soften professional scepticism over time.
A long standing relationship is not automatically a problem, since an experienced auditor genuinely understands a company's business well. The concern is subtler: an auditor who has approved similar figures for many years in a row may find it psychologically harder to raise a serious concern than a fresh set of eyes would.
Rotation brings its own costs, since a new auditor needs time to properly understand a complex business, and that learning curve can itself introduce a different kind of risk in the short term. Most frameworks try to balance these competing concerns by requiring rotation on a cycle long enough to preserve genuine expertise.