In a normal shop, email revenue mostly comes from getting someone to buy again. In a subscription business the buying already happens automatically, so the job changes: email exists to stop people leaving, to rescue the payments that fail, and to make the box feel worth the money in the weeks when nothing arrives.
That means the money is not in the promotional calendar. It is in the first thirty days, the payment failure sequence, and the pause offer you send instead of accepting a cancellation. Get those three right and the lifetime value moves more than any acquisition work would.
It also means your reporting should not be judged on attributed campaign revenue alone. A subscription list with good email is one where average tenure is climbing, which shows up months later.