You press a button an instantly notify one or many people. You can broadcast to them without any hesitation.
It's easy to miss what someone says. It's useful for real-time communication and for sharing jokes. It's distracting unless you turn it off, but people get upset when you do.
You can yell in anyone's ear at any time. It feels bad to miss something. Uninterupted time is rare.
It is Slack and any other chat app of 2017, but it's not their fault.