The flare would slowly fall down after being shot into the air.
The flare in the sky would assist non-night vision players see better & give more nuance to when night vision should be used.
There’s no optic glint at night. There’s no optic glint on FLIR sights either. A flare gun could enable optic glint on regular sniper scopes at nighttime & be an easier way to blind FLIR snipers who are no where to be seen.
(Another post was complaining about players with FLIR on the FAL spawn camping his team at night, unable to locate them.)
Finally, it would help serve as a visual marker (day or night) where smoke grenades can’t be seen by everyone.
If devs fears this being abused at night, have it be a button you press and the flare always goes straight up.