In the US publicly traded companies have a duty to let their shareholders know all material risks to the company that the company is aware of.
The response to this is usually “but everyone knows about climate change”. But that is not the case. By now everyone has heard of climate change but many still don't believe it/dispute it. You can call these people stupid and uneducated but they are entitled to the same protections under the law which means that they have a right to be alerted to the potential danger to their investments posed by climate change. I am sure even the more ignorant would pay more attention if the climate change warnings came directly from an oil company they are thinking of investing in.
Furthermore, the NYAG says he has evidence that Exxon knew it was happening as long as 30 years ago. Back in the late 80s and early 90s climate change was far from settled thing.