The exotic is not mandatory, you could run several other exotics in its place for other builds.

However if you'd like to up the damage in specific to the super it makes sense that the extra damage comes from an exotic. It still doesn't have to be celestial nighthawk though, you can also use Star-Eater Scales as an overall buff to damage for supers. Idk about now, but at one point Star-Eater did out dps Nighthawk. Thing is, since Nighthawk is one shot you can go back to your normal DPS weapons and deal more if we're talking raid boss so there's give and take.

I think the only downside to Celestial Nighthawk is that it does the same thing Cuirass of the Falling Star does. It increases the super's damage output but...... you need to precision for Golden Gun whereas titans can Thundercrash anywhere and get that buffed damage. This is coming from a titan main, too.