An automated email sends itself when someone does something: joins the list, leaves a basket, buys for the first time, goes quiet for four months. The logic decides who gets what and when, so nobody has to remember.
Flows are a small share of what a shop sends and a large share of what email earns, because they arrive at the exact moment a person is thinking about you rather than on the day you happened to write something.
They are also the only part of email that keeps working during a bad month. Built once, tested properly, then left running.