tldr; when a cheap UPS battery is failing the UPS will fail even when the main power is on. I got personal experience with this failure. Maybe the thinking is

tldr; when a cheap UPS battery is failing the UPS will fail even when the main power is on. I got personal experience with this failure. Maybe the thinking is that in case of a power outage you don't want to find out that all your UPS devices fail at the same time. But in this case a 24 hour warning would have been much better.