Somebody bought a 60-day product and got a 30-day email. Somebody else bought a 90-day one and got nothing at all. That happens because reorder timing nearly always gets set once, as a round number, for the whole shop — and nobody goes back to check it against what customers actually do.
The better tools improve on that by working out when a given customer is due to order again. It is still one date for the whole person, though, however many different things they buy: the customer who gets through coffee every month and bought a grinder once ends up with a single compromise that suits neither. We work the interval out per product instead — 34 days for one thing, 90 for another — and time each reminder off the thing it is actually about.
It is the easiest revenue in the building. These are people who already bought from you, already liked it enough to finish the jar, and have given you permission to write to them. All they need is the reminder arriving before they solve the problem somewhere else.