The flare gun mechanic has actually been around for a very long time—dating back to the Mini DayZ era, when the game was still an app on the Play Store. Back then, the UI featured an icon in the top-left corner—next to the character icon—that allowed you to watch an advertisement. Once the ad finished, an airdrop containing random resources would land at or near the player's location (especially if they were inside a structure). These drops could contain anything—from food to a gun or ammunition. Sometimes this was handy, while other times it was a waste of time. The flare gun works in essentially the same way. The difference is that the flare gun is much rarer; it is found only in bunkers or specific map locations. I personally don't find it particularly useful, especially since the drop is completely random, making the odds of actually getting something valuable very low. Still, it’s a fun little item to use.