From Notify a channel or workspace, we can mention many people via:
Everyone in your Slack workspace.
Everyone in the channel.
Everyone are currently online in the channel + notification badge.
Our attentions in a day are limited, and I believe most of people by default snoozed their notifications, so a notification badge usually means something important is happening.
Use @everyone, @channel, and @here for emergency-type reasons. Avoid being The Boy Who Cried Wolf and limit their usage to truly monumental events.
Slack also wrote this excellent guide: Courtesy call: more empathy tips in Slack.