Wine producer may grow their own grapes as well as they may prefer to buy grapes from other vineyards. They buy grapes at a price determined based on the unit amount of grapes. So, the price is proportionate to the amount of the grape to be bought.
On the other hand, high yield is not a positive property of a vineyard from the perspective of wine maker, since the quality of grape improves as much as the yield is limited. So, the grape producers face a unplasant dilemma. They are paid higher if the amount is higher, but the quality diminishes as the amount gets higher.
I don’t have any knowledge about vinification. So, I don’t really know how exactly the limitation of yield affects the quality, but I know that the winemakers who have their own vineyards care about this limitation very much. Therefore, I understand it should be very important for them.