Two weeks ago, we had company for dinner, and a sky darkening storm blew through and killed the power for 1 1/2 hours. There was still *some* ambient light coming in through the sliding doors, and windows, so there wasn't instant total cave like darkness, but we did have the lights, on. The power went out, and so did the lights. Instantly the little power failure lights came on. They stayed on for the full 1 1/2 hours.
IMO, they are safety lights. At 20 lumens, they throw enough light that one may navigate in safety. I think it is generally agreed, that 100 lumens ceiling bounced is a comfortable amount of light for an outage. Therefore they'll offer enough light that you may get to whatever lights that you'll use for your emergency back-up lights, in safety.
The following week our guests returned, and asked... where can they get a power failure light? LOL... the one pictured arrived the previous day. It was a gift for them. I was delighted. Sometimes, you may gift a light to someone and get a "uh... thanks" response. This particular time, they really saw the value in the light, and really wanted one. It made me feel good to show them that we think enough of their friendship, that we went out and got one for them before they asked.
If the power failure occurred 2 hours later, the effect of the auto on lights would have been much more dramatic.