I've found Slack's API integrations to be mostly used as distractions from work, but that may just speak to the team more so than Slack
For us, the API integrations are huge. Slack is our one-stop hub for finding out what's happening throughout the company with integrations for PM tools, Monitoring, Alerting, Deployments, Build Systems, Support Emails, AnswerHub, GitHub, etc.
And of course, giphys.