|You can order this module here|
It is useful when you always know what happens in your system. With the Slack for WHMCS module you dont need to refresh the page every 15 minutes to see that there is a ticket waiting for reply. Now you get instantly notified if something happens on your system.
You can choose on which events you get alerts, also you can post messages to different and multiple channels or persons.
That is the most mature Slack Module for WHMCS. It does not only look nice, it also has some cool features!
Unlike other modules, this module does not post the message directly to slack. It does maintain a message queue. So you ask why that is important?
Because if you send messages directly to Slack, there will be a small delay at the page load time for the customer. Your WHMCS Installation will wait until Slack processed your request.
Much worser it is if the slack service is not reachable from your server. Then your clients will have a long page load time delay until the request time outs.
With this module you get your own sending API. You can send messages from your own scripts to the slack channels and also can integrate external services with your own, easy to use, HTTP based API.
The HTTP based API can be useful if you want to integrate an external monitoring service (like Zabbix or Nagios) into slack.