To add to others’ posts. It can be a huge variety of things that risk making the service unstable, unresponsive, and worst case could corrupt data in flight.

Customers view scheduled maintenance as minor inconvenience. Unplanned outage as an annoyance, and loss of data as a dealbreaker.

So any time there was a chance that what we need to do would limit functionality - or otherwise make the system unstable - best to take the system offline for scheduled maintenance.