Complaint letters and Praise letters.
Complaint letters are problems. When a customer makes a complaint either on the phone or by sending a letter it indicates there is a problem. If several complaint letters are made about the same thing, then it is obviousely a problem that needs to be sorted immediately. Organisations record complaint letters and check to see if there are any common factors.

Praise letters which thank staff for good service or a product is to be treasured- Which is why organisations pin up theese letters on a notice board or mention them in newsletters.