A listing is permission. A facing is the actual business.

Then the shelf does what shelves do. A neighbouring brand's rep tidies their block outwards. A promotion rebuilds the bay and your facing does not come back. A store runs a gap for three weeks and the replenishment system quietly learns that the line does not sell.

None of that appears in your sell-in numbers until the category review, when it appears all at once. Somebody has to be in the store. Either you pay a field force to do it, or you work with a distributor whose people are already walking that aisle for other reasons.