Pruning is good for shrubs and plants for many reasons. It helps to shape a plant or shrub, and to keep vigorous plants, such as Wisteria, in check. It is also a opportunity to keep the plant healthy by cutting out damaged branches. With some plants pruning will help the shrub or plant to produce more flowers. In the wild plants don't get pruned, but then they can become a tangle and a garden is, to one degree or another, trying to make order of plants, cultivation to tame the nature of plants. You only have to see how vigorous are some climbing plants to appreciate without a good prune they would take over.
Some variegated shrubs may lose their variegation on certain branches and pruning those out will help to keep its colour. This is called "reversion". It occurs when the variegated branch only produces green leaves and it is best to prune off t the non variegated branches.
Pruning is can perplexing, what and when to prune. It helps to bear in mind that pruning will not usually kill a plant, unless you prune very hard and cut hard into the wood, especially with plants such as lavender, Wisteria, Artemisia . You may sacrifice flowers if a shrub or plant is pruned at the wrong time. Why is this? If you prune a plant at the wrong time you may cut off the very branches carrying the dormant flower buds, which is why timing is so important. If you are not sure of the identity of a shrub or plant, and so cannot decide whether or when to prune it, send a quality image to The Sunday Gardener for advice on what it is and when to prune. Free plant and shrub identification service.
It is also true to say that some of the more difficult plants to grow, tend to be the ones with essential pruning requirements. Wisteria will eventually stop flowering unless pruned twice per year which makes pruning essential. To check out how to grow and prune wisteria, details are on the growing Wisteria and there are three videos: how to do the summer and winter pruning, and how to make your wisteria flower.
Clematis have fussy pruning requirements as well, for all but the early flowering varieties. If you don't like pruning, the best clematis for you to grow are the early flowering types such as Montana, Alpina, Cirrhosa, or the slightly tender, sweet smelling C.Armandii. These early flowering Clematis do not need any pruning. Clematis fall into three groups which decide when they should be pruned, the early flowering Clematis are group 1 which do not require pruning but Groups 2 & 3 do; full details are on the growing Clematis page and videos.