Because it's such a popular communication channel, it has wide-ranging integrations with plenty of other tools. You can tack on a seemingly unlimited amount of integrations and Slack applications to make the tool even more powerful -- for instance, many teams will pipe in A/B test results, analytics notifications, new customer or transaction notifications, or even sales or customer support bots. While undeniably powerful, Slack is easy to start using immediately.
With offline marketing, it's very difficult to tell how people are interacting with your brand before they have an interaction with a salesperson or make a purchase. With digital marketing, you can identify trends and patterns in people's behavior before they've reached the final stage in their buyer's journey, meaning you can make more informed decisions about how to attract them to your website right at the top of the marketing funnel.