Here are a few good, natural reasons to prune.
Why prune? Seems a funny question, but is worth some consideration. The look, the aesthetics, are very important to us and most of us know what we want our tree to look like, but your tree has a natural shape and, given a chance, it will probably look pretty good. The old European practices of complete tree control have mostly slipped away. The techniques and time for the extensive labor are a thing of the past. Some of this in the right place can be very attractive, and I really didn't appreciate formally shaped trees and shrubs until I saw many gardens in England.
Know this, trees don't like to be shaped, they will fight you all the way, and the regular loss of foliage coupled with many pruning cuts to accomplish the work does not have a positive effect on the tree's overall health. A cotoneaster hedge, for example, is made of plants that, if planted singly, each would in 25 years or so form a 15-foot ball. A hedge is severely compromised, doesn't thrive, it survives, compared to the same species planted singly or in groups. This is the main reason why, when a serious problem like oystershell scale shows up, it ravages most hedges; they are weak. One technique that I recommend and that works for a struggling hedge is to quit pruning. Shifting the balance slightly towards having more leaf mass, to generate food, energy for the hedge can make all the difference, allowing the hedge some extra strength to fight back.
Anyone with a plant or plants that they shape each year knows how seriously plants take this challenge. They feel a loss of leaf mass, which in a naturally growing plant is in balance with root, trunk and branch growth. When plants perceive that loss or imbalance they dedicate a lot of effort to making sure that the lost leaf mass is replaced, and then some. You shape, cut back branch tips; the tree grows them back. Stimulated by these practices, the branches grow at a faster rate than is natural.
Here is probably the right place for a serious confession. I like trees in as close to natural state as possible, which interestingly is the state where they are the healthiest. Plant the right size of plant for the site, and let it be.
So why prune? Maybe you like to see your tree with branches right down to the ground; that’s OK. Maybe you like it so even basketball players can walk freely underneath. That's OK too. Use an aesthetic that feels right for you. There are extremes. Sometimes you will see a spruce tree that has been pruned to copy the form of a palm tree, not a great solution. Except for the spruce example, most forms usually look all right and don't seriously harm the tree. It is the complete reshape, repeated each year, that frustrates your tree.
There are some very good reasons to prune, health benefits that come from good practices. The removal of deadwood is a major health benefit, greatly speeding up the time (in years) usually needed to naturally remove dead branches from the trunk. Once the dead branch has been removed the bark tissue is now unimpeded to grow over the wound, naturally closing a fungal infection site.
Diseased branches should be removed as soon as they are seen. This is the difference between keeping a healthy tree or, if ignored or unseen for too long, a tree to remove. There is nothing you can do to prevent your apple tree from being exposed to fire blight. There is an ignored blighted tree right around the corner that the bees and birds visit and then return with the bacteria adhering to their feet. This will be a test of the strength of what I will call its immune system, very different from our own, but the term works. It is the unseen, unnoticed, ignored disease that two or three years later has destroyed half of the tree, leaving no option but to remove. I see these all too often.
Damaged branches are another thing to keep a eye on. Usually caused by storms, there are broken branches high in the tree that we call hangers. These can let go at any time and someone should go up and prune. Many times the branch has already broken free but is caught in another branch. These also are unpredictable and should be removed. Trees suffer from too much branch removal, and a healthy, still functioning branch that has changed position need not automatically be removed, if it is no danger. Your call; as I said, a grey area, needing a clear eye.
Some trees, especially the cultivated flowering crab apples, are very good at generating new shoots along branches in the interior of the tree. Left too long, the airy spacious look disappears. If left for a period of years these fast growing shoots will get seriously tangled above in the tops of the tree. Depending on the overall health of the tree, I usually remove most of them. Sometimes I don't care, if a shoot has found its own space in the upper crown, and doesn't look too bad. Other times that I leave them is if a tree is weak or failing. This is a repeat of the energy from leaves theory.
If a crab is infected with fire blight and has suffered a loss of leaf mass, this is another time to leave the shoots alone until it recovers. They can easily be removed in a couple years once the tree recovers from the disease. Don't think that you have to check all the boxes of pruning theory with each case. There are times when the energy produced in the shoots and leaves is a real benefit to the tree. Be kind to a sick patient.
These shoots go by a variety of names, shoots, suckers, epicormic branches, a technical term that is most accurate. It means upon a branch. Call them what you want, we will know what you mean.