Another would be a manufacturing firm with thousands of sensors sending data to the database continuously, latency and delivery mechanism mean that timestamps often arrive out of order. There is a case study we did with CNC machines manufacturer DATRON [3].
Tracking moving vehicles (say cars, ships, drones, planes) on a map where the two primary axis are time and space - if you want to see the position of those vehicles in the past, you want to have the timestamps ordered.
Finally you can think of typical devops/monitoring/alerting use cases. Verizon uses QuestDB to monitor metrics for autoscaling decision to their Vespa engine that provides search, recommendation, and personalization to their users [4]. Here you want the series to be ordered by time.
[1] https://levelup.gitconnected.com/tracking-multiple-cryptocur...
[2] https://www.tradersinsight.news/ibkr-quant-news/optimizing-t...