The Nairomi plot soured the public's perception of Superman. Some people thought he was a monster for heat visioning a bunch of rebels in the desert. And some people, rightfully, began fearing him. The story the media went with snowballed that fear into hate. And people, like Batman, started wondering if Superman were to take things into his own hands what's stopping him from burning the place down.
Lois' investigation not only cleared Superman, but went on further to prove he was setup.
What this does is frame Superman's death as a heroic sacrifice instead of an act of redemption to the public. And that's why we get the #MenAreStillGood scene in the epilogue.
Lois represented the part of humanity that refused to believe Superman was a bad guy. Granted, she has a vested interest in seeing Superman exonerated, but it's an important role she played nonetheless.
An aside of the investigation squarely pins everything on Lex. Maybe the USDJ would have gotten something on him eventually, but Lois had the "smoking gun", or in this case bullet, needed to put him away.
This is just what she did BvS. If I were to break down her significance in MoS I'd be texting on my phone all day.
Compare that to Margot Kidder in the Donner films...