The job it replaces
Without one, running a channel means watching it. Someone has to remove the scam link, greet the person who just joined, post at the hour people read, and notice that last Tuesday was unusually quiet.
A manager does those four things on a schedule and tells you afterwards. The work does not disappear; it stops needing a person awake for it.
Moderation, which is most of it
Spam filters, link rules, flood limits, CAPTCHA on join, warnings and bans. Each is a module you switch on for a specific channel, with its own settings, and each one logs what it did so you can check whether it was right.
The honest caveat: a filter that is too strict removes real people. Start loose, read the log, tighten.
The parts that are not moderation
Welcome messages, scheduled posts and broadcasts, auto-replies, triggers, polls. These are the ones that grow a channel rather than defend it, and they are the ones most people turn on second.
Analytics sits underneath: member growth, message volume, which posts moved anything.
What it does not do
It does not write your posts, and it does not make a quiet channel busy. A manager removes the reasons people leave and the chores that stop you posting. What you post is still yours.
What it takes to start
A bot added to your channel as an admin, and a decision about which modules you need first. Moderation and a welcome message cover most of the first month. The full list is on Modules; the free plan has no limit on members or messages, and the tiers are on Pricing.