The neglected tree
Once again, borrowing from Stephen Hayes from Fruitwise:
Year x - at some stage, obviously the tree will reach its maximum potential size (depending on the variety, rootstock, soils, care, disease, rainfall, nutrient etc etc), and the centre of the tree will fill up with growth, like a hedge. As this happens, less light penetrates into the centre of the tree, so there will be less fruit and it will tend to ripen poorly and be small. Also, less air circulates therefore fungal disease is more likely-fungi prefer damp and dark conditions. The tree will therefore fruit less and be subject to more pests and diseases. This happens in nature and in very old neglected trees. It is not necesarily a disaster, such trees can be very beautiful and can build up their own ecosystem. They just don't fruit so well. The tree for its own sake only needs to reproduce a couple of times in its life to keep the variety alive, so (speaking as if the tree had a mind of it's own) it doesn't mind if it only produces fruit every other year and if that fruit is small and unripe.
Again, the big old un-maintained, lichen-covered tree in your garden may provide a home for the birds and a beautiful architectural feature in your garden and give 2 or 3 baskets of apples most years, and if you are happy with this there is no need to do anything except get up a ladder (careful!) once every 3 years and saw out a few diseased post-mature branches to allow a bit of light in and space for a bit of new growth. You however, may well prefer a regular supply of quality fruit in which case you are going to have to manage the tree and regulate its growth to some extent. Thats where pruning comes in.
Supporting Video Clips
Please select from the Fruitwise videos - restoration of neglected trees: