Slack is facilitating the creation of many, many “invisible” apps. https://slack.com/apps
Howdy is a good example: http://howdy.ai/
Strewth (made by us) is another: https://strewth.co/
In general, offloading functionality into bots that can prompt you and interact with you in interesting ways leads to so many possibilities. It allows the creation of value without investing in a new UI, significantly reducing the complexity of building new apps.