A mounted flashlight is INSANELY bright. Like, a lot of soldiers have a nickname for them along the lines of “Sun-on-a-stick”, so I reckon if you swept a person or animal with it at relatively close range you’d be able to daze them, make their next action take longer.
Apart from that, aiming is kind of faster, like you can snap on to a target faster, but anything other than a snapshot isn’t really affected.
The key use of underbarrel/heavy duty lights, being able to see shit in the dark without having to put away your weapon doesn’t really matter in cata right now, because you don’t need to wield a flashlight to have it provide light.