There are several reasons to prune well. But don’t take my word for it. Head over to a YouTube recording from the RHS, for example, and you’ll see that timely pruning can save a tree from infection, disease, and pest invasion. Pruning at the right time of year needs research. I can do that for you; take a look at this page I have researched from the charity RHS:
Unless dealing with young trees or saplings, it is best to consult a tree surgeon or arborist. Fruit trees are best trimmed at different times of the year. Pear can benefit from a pruning in July, whereas apple may be best pruned in March. Do your research, and workout why you are pruning. Some pruning will encourage new growth. For example, if you take out lead branches you will end up with more lateral growth. Often trees benefit from a prune when dormant. Consider wildlife. Our birch is best pruned in summer, but in August some wood pigeons were nesting on one of the branches. We managed to prune the nearby branch without casualties.
Thinning is perfect for mature trees; taking out several branches and opening the tree up into the shape of a vase. Rember that thinning will produce new growth. Of course, how you prune can depend very much on the current shape of your tree once established.