The main reason is to use a chat solution is as a unified ledger / single pane of glass. Silly as it is, there isn't really a better solution out there for "integrate all of my third-party deploy, CI, build, etc. status updates in real time, in one place." Sure, someone can click into GitHub, but if you use another service to deploy into, does GitHub pipe the status of that service into your dashboard? How about error logs and alerting, do they go there? If there's a customer incident, does the trust site status show up as well?
On Slack and other chat solutions, it's possible to set up a #operations style single-pane-of-glass channel with deploy notifications, error alerting, and customer communication platforms all pushing to the same place. If an incident occurs, engineers, product, support, etc. can all collaborate around what's going on in real-time without needing to ask "hey has someone updated the trust site yet?" or click into 10 different tabs.
It's honestly pretty good when it works well and really bad and noisy when it doesn't, but it has a place.
Non-engineering are usually in Slack too, which really helps when support, product, or the field need quick answers to easy questions like "has this commit been deployed yet today?"