A ticket is a chain of custody. Device, model, fault, condition on arrival and what the customer agreed to has to be recorded at the counter, because a dispute about a pre-existing crack cannot be settled from memory.
Parts stock ages badly. Screens for a handset three generations old occupy money and shelf space nobody will spend, so knowing what actually moves is what stops a drawer becoming a museum.
Warranty needs the history. When a repaired phone comes back six weeks later, knowing which part was fitted, by whom and when decides whether it is a free fix or a new job.
Devices get abandoned. A repaired phone nobody collects occupies a drawer, carries somebody's data and eventually raises a legal question, so a policy with reminders and a stated deadline is not optional.