You stop planning the next period from memory alone
Before, you decided what to restock, push, or cut by remembering what had worked, which tends to vary from person to person and month to month. With the habit of always starting from the most recent period's review, every decision gets the same origin: a number, a note, or a recorded observation. That cuts the odds of repeating last month's mistake without noticing.
You record every decision with its evidence attached
Before, you often decided to push or cut an item in a quick conversation, without keeping track of why. With the requirement to point to the number or record backing each line, the plan carries its own justification. That makes it easier to explain the decision to someone else on the team or to check whether it still holds up next period.
You add fewer made-up lines and redo less mid-cycle
Before, you added items to the plan without real evidence, and often had to undo those decisions halfway through the period, once it became clear the basis was just an impression. With the rule of leaving a line out of the plan until the evidence shows up, fewer decisions need reversing after they are already underway. The store spends less time fixing a plan that should not have gone in that way in the first place.
You repeat the same planning cycle period after period
Before, you started every plan from scratch, depending on whoever remembered what had been decided last time. With the plan written in three sections and tied to each period's review, the next cycle gets a clear starting point: check what was decided, compare it with what happened, and restart from the newest review. That makes the planning less dependent on one person's memory.