This title is misleading. The article doesn't say that the chat history will be used as evidence, only that it exists. Whether it can be used in court is an unsettled question, as explained in the last few paragraphs.
a lot of people, especially younger ones, seem to use chatgpt as a neutral third party in every important decision. so it probably has more extensive records on their thoughts than social media ever did. in fact, people often curate their Instagram feeds - but chatgpt has their unfiltered thoughts.