The classic example we already have of this is when you are stopped at a side road about to enter the main road, and a car coming towards you on the main road signals to turn in.
Many people take the fact the other car has their turn signal on as a guarantee that it's safe to emerge, but any good driving instructor will tell you to wait until the car actually begins to turn before you yourself emerge.
They had their signal on but that doesn't mean they're actually going to DO what the signal said they would.
Same with the front brake light. It would be like "Well their front brake light came on, so I assumed it was safe to step into the crosswalk" NO. They could have just tapped the brake a second, doesn't mean they saw you, or they will actually stop.
And the power switch was like KA-JUNK when you pushed it, because it was a big ol' switch that actually physically connected and disconnected the power.
"It's now safe to turn off your computer" went away after we moved to software power control, where the operating system could signal the power supply to turn off.