I guess that's a fair point. It cuts both ways, but given that so many people use Slack as opposed to talking, the exact words people used and when are could be open to view. Whereas, before all of this, you may only just have the minutes of any official meetings. Any side chatter not in the meeting room and/or exact phrasings would be lost to time.
Slack is also used because it allows to create persistent channels that are searchable. So they often end up being a knowledge base for the company.