It's a central place for information with good search, the ability to ask people to answer, Slack integrations, reputation (always a bonus), and the ability to link to easily other documentation sources.
Being able to follow 'tags' for the terms that apply to your teams is very valuable, as well. You can link them to Slack channels (so they are automatically posted), or just receive emails when someone asks or answers questions.
Establishing reputation for your answers is also great - it provides incentive to provide good answers, ask high-quality questions, and edit answers or questions that need just a bit more clarification or adjustment.
The high quality search on Stack Overflow is nice, too. If you lack a central search mechanism, then Stack Overflow can be a nice bridge between different sources of documentation (whether static sites, wikis, tickets, etc). Review collected by and hosted on G2.com.