It's nice having a little margin so if things don't go to plan you aren't left holding the bag.
How often has it been called into action? The issue I typically see is that people test under the best conditions, then when things actually happen it's not as clean and pretty as your test scenario. For example, you forgot you had a long running file copy open in a terminal, so the timeout waits much longer to kill things, or if you are there onsite watching it you frantically try to log in to close things down, all the while the battery backup is beeping faster and faster. The wifi access point you were connected to is on a different battery backup, just a small one, and it dies as you are logging in with a laptop that hasn't been charged, and is taking a painfully long time to boot up, your primary PC is already shut down, and you can't find a patch cable long enough to reach the stepstool you are balancing the laptop on.
Having an extra 5 minute cushion can mean the difference between a clean shutdown and a 24 hour parity check.
Can you tell I speak from first hand experience? 🤣