The Malfeasance exotic hand cannon you win at the end of Darkness in the Light Destiny 2 quest can really improve your gameplay. It will not only come in handy during PvP modes within the Crucible when your skill and ability are duly tested, but also offer an advantage during PvE missions such as strikes and Nightfalls.
Moreover, the gun can also be a helpful companion during higher-level content in dungeons and raids. We would say that it is one of the most underrated weapons in the game. And hence you should complete the questline as soon as you can during your playthrough.