A finding without a reference is an opinion. Recording which criterion failed, on which page and how severely is what lets a developer fix it and an auditor accept it, and a prose list of problems achieves neither.
The retest is where projects overrun. Verifying a hundred fixes takes real time, some will not have been done properly, and a studio that quoted only for the audit ends up giving that week away.
Somebody else will read the evidence. A regulator, a client's legal team or a tender panel may examine the report, so screenshots, dates and methods have to be recorded as they were, not reconstructed later.
Sites decay after sign-off. Each content update can reintroduce a barrier, and a studio that offers periodic rechecks turns a one-off audit into a relationship that keeps the client compliant.