Customer Satisfaction (CSAT) surveys let you measure how customers feel about the support they received. Enable them per help desk under Configuration → CSAT.
Once enabled, satisfaction results feed your reporting, so you can track quality over time and see it broken down by agent or group. This turns a vague sense of "are customers happy?" into a number you can watch and improve. Because CSAT is opt-in per help desk, you can run it where it matters and leave it off elsewhere.