Imagine... your Fire Alarm System goes into fault. No obvious cause. An engineer arrives, opens the panel, tells you it's all clear... and ten minutes after they leave, the fault comes back.
That's not a scenario we made up for effect. It's the kind of call-out Fire Alarm engineers deal with in the real world and it matters because a panel showing "all clear" doesn't always mean every part of the system is genuinely fault-free.