A grape vine in its natural habitat given ample water supply and nutrients is a vine which likes to climb but produce small bunches of tart grapes. It would rather use most of its energy making leaves and shoots. The shoots enable it to attach itsself to other plants or it can develop new root systems if in contact with the ground.
In short, a grape vine left to itself isn’t ideal for growing grapes. It takes cultivation by a farmer in order to create the conditions which produce an abundance of sweet grapes.
A friend of mine and her husband own a vineyard, and I remember her talking about the need for grapes to be under stress. Vines need to struggle in order to produce good quality grapes. But when the farmer puts the vine under stress, by making its nutrients scarce, restricting water, and pruning them back hard, and crowding them with neighbors, it focuses its energies on making grapes. Instead of devoting its effort in increasing its own size, it instead focuses on reproducing itself. That means making grapes. But in this process, the farmer must be careful not to over stress the plant. Over stressing will prohibit growth, grape production and makes it more susceptible to disease. So the farmer must carefully cultivate just the right environment which brings maximum fruit production.