Darkness happens. It happens every day, in fact (well, depending on where you live-in some places it does not get dark for long periods of time and in others it stays dark for long periods of time), so it is something you will have to face, especially if the grid collapses. Your eyes will acclimate to the dark to some extent, but there will still be more you cannot see than that you can.
Seeing in the dark is important because there is so much you need to see to survive. In an urban area after the collapse of civilization, a lot of danger will be present. Broken glass from windows as well as discarded items can trip you as you move about. In a wooded area, holes or downed tree limbs present another form of trip hazard. While it is important to know your terrain, that only goes so far if you unexpectedly wind up somewhere unfamiliar.
Flashlights are essential in any BOB and keeping one on your person is ideal as well. However, a flashlight is only good for a direct beam and does not have the best light projection for other purposes. If you want a camp light, a fire will work, but it also makes you visible from afar. You also can't put a fire out with the flip of a switch when (not if) you realize you're being watched. You have to douse a fire, which will not only take time but will also take precious water you may not have to waste. The fire may also put off steam or smoke when doused, continuing its telltale announcement of your position.
What you need is something with good light output in addition to being able to switch on and off quickly. The answer to this quandary is actually much simpler than you might expect. You can achieve this goal with a head lamp and a jug of water with minimal effort or assembly. If there is not a headlamp in your BOB or stored in your car, be sure to purchase one. It will be useful for many purposes and has the advantage of being hands free. Also be sure to store a jug of water in your vehicle or have one accessible some other place you will be able to get to it in a hurry.
To create a diffused light effect that will cast directional light, affix your headlamp to a jug of water with the light source facing into the jug. It will light up the water and jug, creating a sizeable area of illumination. If you need to douse this light in a hurry, simply switch it off and there is no telltale sign of your location. When you need to use it again, it will be ready and waiting. Your water will also be available for drinking rather than wasted on putting out a fire. You also don't have to worry about accidental spreading of fire as there will be no fire to spread if you use the headlamp water light.
This is great for inside of a vehicle or a home, both when the SHTF or even if you are simply having car trouble at night and waiting for help to arrive. You could even change your own tire by this light. Regardless of how you use it, remember that the option exists; you never know when you might need it.