Nahida’s C2 is an incredibly potent Constellation that raises the damage ceiling of her teams by a noticeable amount, and lets her work well in unconventional uses such as Eula teams. However, C2 is not a “must-have” at all, as Nahida’s kit is already very powerful and fully complete at C0.  C2 does not add any significant utility to Nahida’s kit, but simply rewards players who highly invest into Nahida’s Constellations for larger damage numbers. That said, it can allow her to slot into teams she would otherwise be highly suboptimal in.