I am an Asana fanboy for sure, and really like Slack as well. But not everyone uses a project tool in the same way, nor approaches it as project managers do. Some do not care about project tools and feel they get in the way of getting work done (more work about work). Some might even say they are too busy to manage yet another tool as they are already using several tools to get the work done.
One might, as I do, argue this is the reason Asana has integrations so you can work where you are while allowing Asana to manage across platforms.
Here are a few reasons you should start using Slack with Asana.
Team Notifications
Often there are folks, or groups of folks, that do not adopt a project tool as others do. So they are not in it 24/7 as say a PM/Manager would be. They already have a suite of tools, such as Slack and do not need Asana for many of its features. They are the Doers, not Trackers.
For these types of groups, having notifications of new tasks added or task coming due or blocked is a great way to be sure everyone has visibility and someone can action that task even if they are not the assignee.
Outside Teams
Not everyone is using Asana - SHOCK - I know 
the larger issue is some companies do not want to give licenses out to those who just need visibility but have no tasks. Asana does not have a way to report out to these folks unless you adopt a 3rd party app (more cost, more noise, more management). Messaging out to a group in Slack for these teams can help everyone stay informed of progress without physically creating a deck to share out.
Escalation
Are there tasks more than x days past due? Slack a leadership/executive channel. Sure, you can @mention them, but let’s be honest, you get quicker responses in Slack especially when people know others have visibility also. Also, when people know that tasks will get escalated, they tend to care about their completion more.
All Roads Lead to Asana
I have a saying that if it’s not in Asana, it doesn’t exist. As a PM, I wont’ track tasks outside of Asana. But I also very much understand that not everyone sees the same value of Asana as I do. So I meet them where they are at- in Slack (or InVision, or other tools). This allows quicker adoption while making sure updates and communication is transferred back to Asana.
Personally, I want to create tasks where I conceive them. If I think of an idea while I have Slack open, then I want to ideate right then. If using my browser, then clicking on the extension is where I ideate. Now in Zoom, I can easily create/update task right from a meeting. More roads leading back to Asana.
Creating, updating, interacting with work in many applications allows all roads to lead to Asana. Without these integrations and capabilities, people would silo and PMs would lose track of work. This is just human nature.
Slack Integration Functionality Needs to Grow
So I say meet people where they are at and grow the capabilities of Slack. Allow for better task creation, reporting out to channels with weekly dashboards, and project statuses to show in Slack as it does in Teams.
I do not see myself using another project tool anytime soon, but I also do not see the need to force everyone to my opinions when there is a way to integrate with apps they are already using.
