To keep them at a manageable size.
Pruning is done to direct the energy of the vine into producing fruit instead of stems and leaves.
Prune to keep the fruit growing close to the main stem. This way the sap doesn't have to travel far to produce fruit.
In order for fruit to ripen, pruning needs to be done in order to let in sunlight. Unlike most other fruits, grapes do not continue to ripen after being picked from the vine. Thus, it is essential for the grapes to get enough sunlight on the vine to fully ripen.
So there are apparently many reasons for pruning. Pruning rejuvenates by allowing the vine to produce new growth rather than spending its nourishment on gangling branches that are no longer fruitful.
In chapter 15 of John Christians are compared to the branches of a vine, Jesus used pruning as a metaphor for the way God interacts with us: "I am the true vine, and my Father is the gardener. He cuts off every branch in me that bears no fruit, while every branch that does bear fruit he prunes so that it will be even more fruitful" (John 15:1-2).
There are times when God ends an endeavor that has been truly fruitful. It can be confusing and even make a person second-guess the good work that has been accomplished or even doubt God's faithfulness. So one may ask, “If a work is bearing fruit, why would He allow it to end?” We may think that just because something is good God will allow it to go on forever. But you see, God is the one who "changes times and seasons" (Daniel 2:21). He prunes us and everything in our lives, from our relationships to our occupations, so that they can become even more fruitful in a new season. Jesus said, "I am the vine; you are the branches. If a man remains in me and I in him, he will bear much fruit." (John15:5) Apart from him we are like branches that wither, needing the vine but having turned from the source, cutting ourselves off from the gardener.
