The calendar has to be seen across accounts. Sends stack up on Tuesday mornings and Black Friday week, and an agency without one view discovers the collision when two clients are both waiting for approval.
Addresses rot quietly. People change jobs, abandon accounts and stop opening, and a list that is never cleaned slowly increases bounces until the sending reputation suffers for everybody on that domain.
One careless send does lasting damage. A large campaign to an unengaged segment produces complaints that take weeks of careful sending to undo, so five minutes of checking before the button is pressed saves a month of repair.
Monthly totals hide the truth. Revenue attributed to each flow and each campaign separately is what shows that the welcome sequence earns quietly all year while the newsletter everybody agonises over earns almost nothing.