(The NSA clearly doesn’t want me giving you this information, because I typed in an answer than then it disappeared. They may have mind-controlled my cat and got it to select all and hit the space bar, but Undo didn’t work, so it must be their black ops cyberwar unit.)
It is marketing crap, but not entirely.
It makes people think they “are tactical” or have some advantage when they have one.
In practice, they may have some uses. They’re generally crazy bright and have a “strobe” feature that (in the imagined situation where it is dark and evil minions want to harm you), you can use this not just to see people (the better to e.g. shoot them), but also to blind them , startle them, disorient them, etc.
(Of course, they may not figure that people such as my friends used to use strobe lights in the dark when playing “savage melee combat”, in which case we may be fairly used to fighting during a strobe effect, and it may actually trigger us to enter a battle fugue state…)
Others have an infrared mode, so with the right filtered vision gear you could also see people without them seeing you. With both types, maybe you could massacre a room full of goons as demonstrated pretty well by Hit Girl in Kick Ass as seen in the scene where she attaches a tactical flashlight to her pistol, uses it to blind/confuse people while shooting them, then the scene shows how it can also give away your position and draw attention, but then she sets it down pointed at the gunmen, getting them to just see (and continue to shoot at the flash), and uses that and cover to get around behind them and kill them.
Or you could get a Maglight and use it as a club to bash people.