Clarifying your pruning goals will make you a better pruner and reduce your confusion over what to prune.
Often when people look at a tangle of branches, they can't make sense of the tangle of branches and just start cutting anything.
Instead, carefully look at your tree or shrub and figure out what your pruning goals are. Then look again to identify just which branches need reduction or removal. Every branch you remove should be to reach your goal.
Developing Strong Trees is a very important reason to prune. Due to research in the last 20 years, we know a lot about why trees fail (that's arborist talk for breaking or falling down) and how to train young trees and prune more mature trees to minimize failures. This pruning is vital, so make sure to make this your pruning priority with your trees.
Encouraging or discouraging flowers and fruit is a common pruning goal. We want to encourage bloom in plants with showy flowers such as: roses, gardenias, hydrangeas, and loropetalum. In fruit trees and vines we want to promote flowering so the plants will set fruit. And then there are some plants with stinky flowers (thankfully, not many) or messy fruit where we might want to discourage flowering and fruiting.
Reducing the size of the plant is another important reason people prune. Plants look so tiny when they are young and they get planted too close to each other or to a wall or walkway. As the plants mature, they may grow to be larger than can fit. Fruit trees usually grow larger than can be harvested without a ladder. Reducing the height lets you reach the fruit with your feet safely on the ground. Vines must be pruned to keep them from taking over nearby buildings or trees.
Adapting the shape to suit your needs. Sometimes you need a shrub to be narrower so you can walk by it. Or a tree to start branching high enough so that the garbage truck won't hit it as it drives under. Looking at the form of the plant may give you an idea on how to prune it to suit your needs.
This Saucer Magnolia was planted many years ago and it had grown to encroach on the entry to the restaurant. Instead of removing it, the side branches were removed and the canopy raised to allow people to walk by without sacrificing all of the spring bloom.
Enhancing the beauty of a plant by judicious pruning. This mature Japanese Maple was carefully pruned when young to enhance the natual layered appearance. Small twigs that grow into the open areas are pruned every year to maintain the pattern.
Just for fun. Some plants are tough enough to be pruned into whimsical shapes. Topiary is the art form where plants are sculpted/encouraged to grow into shapes such as teapots, bunnies, spirals, or other whimsical shapes. It does require frequent pruning over many years.