Why is having a complaints process important for a CSO?

Honestly I’ve always thought of complaints processes as something you put in place when things go wrong, and since we haven’t had any serious incidents I’ve never prioritised it. But a donor recently included it in their due diligence checklist and it made me wonder if I’ve been thinking about this the wrong way. Can someone help me understand why a complaints mechanism is actually valuable for an organisation that’s genuinely trying to do good work — beyond just being a box to tick for funders?