A grapevine needs pruning for lots of reasons, to make the vine easy to look after and to reduce the crop load so the fruit can fully ripen.
Maintain vine form. Vines are originally forest dwelling climbers! That means left to their own devices they would sprawl until they found something to grow up. In a grape growing vineyard we train them along trellis (post and wires) so we can pick the grapes at harvest, but also to open up the leaf area so we can increase airflow and sunshine and minimise disease (this is often referred to as canopy management).
To reduce the size of the crop. Left unpruned a vine would try to grow a lot more fruit than it could possibly fully ripen. A grapevine's goal isn't for its berries to be turned into delicious wine, but for those birds to eat their berries and spread the seeds. Birds will happily devour fruit that is less than ripe, but vintners need the grapes to perfectly ripened so we can make perfect wine.
Selecting where the fruit should grow. We select which canes or buds to keep and how they will be spread out along the wire. This means trying to avoid clumping of fruit which can create disease and to maximise their exposure to the sunshine that is coming (we hope!).
The picture above shows Will (left) and John near a spur pruned vine. The spurs are laid along the wire and kept there for years; this means spur pruning involves tidying up the canes that grew over the last summer. We renew spur pruned canes from time to time to improve yield - and you can see that we've done that in a previous year with the slightly thinner spur on the left of the vine Will is in front of.
The other common method of pruning is cane pruning, where new canes are laid along the vines each year. We only cane prune our Sauvignon Blanc which can more easily ripen a heavier crop load which is often the result of cane pruning. In the photo below you can see the new and much thinner canes laid down.