Pruning is a necessary evil, especially when transplanting. It happens that whenever the demand for moisture in the foliage or top part of a plant becomes more than the roots can handle it will die. When the roots are cut back for transplanting it follows that the top of the plant must be cut back an equal amount to maintain a balance in the plants system.
Another reason for pruning is of course to keep the plant from growing beyond a desired ornamental form. Bonsai trees would be an extreme form of this. Or as in the case of Rose vines, a desired flower may only bloom on new growth so the plant is pruned back to get an optimal amount of new growth and flowers. Pruning of flowers on a plant can also be used to encourage a plant to focus all its energy on growing one giant flower or fruit/vegetable. This is called disbudding.
When pruning use sharp tools. For a plant use a single bladed shear and for larger woody stemmed shrubs and trees use a saw. Prune with the blade angled toward the plant and away from the stem. This way you will only bruise the part of the plant that you are cutting off. Good luck and happy pruning.